Title of Invention: ULTRASOUND DIAGNOSTIC APPARATUS AND ULTRASOUND DIAGNOSTIC METHOD Abstract: An ultrasound diagnostic apparatus includes: an ultrasound probe including a plurality of ultrasound transducers; a device for determining optimum sound speed values at a lattice point set in a shallower region than a region of interest (ROI) and in the ROI on the basis of an ultrasound detection signal outputted by the ultrasound probe; a device for arithmetically operating a received wave received from the ROI on the basis of the optimum sound speed value; a device for setting an assumed sound speed in the region of interest, obtaining a received wave received from each lattice point on the basis of the assumed sound speed and the optimum sound speed value at the lattice point, and synthesizing received waves at lattice points to obtain a resultant received wave; and a device for determining a local sound speed value in the ROI on the basis of the received wave and the resultant received wave. Detailed Description: CROSS-REFERENCE TO RELATED APPLICATIONS The present application is a continuation of U.S. patent application Ser. No. 12/585,482, which was filed Sep. 16, 2009, which claims priority from Japanese Patent Application No. 2008-245519, filed Sep. 25, 2008, and from Japanese Patent Application No. 2009-088363, filed Mar. 31, 2009, the contents of all of which are herein incorporated by reference in their entirety. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrasound diagnostic apparatus and an ultrasound diagnostic method, and more particularly to an ultrasound diagnostic apparatus and an ultrasound diagnostic method for taking an ultrasound image of a subject using ultrasound and displaying the ultrasound image. 2. Description of the Related Art Conventionally, attempts have been made to measure a sound speed value in a part (a region to be diagnosed) in a subject (hereinafter referred to as a local sound speed value) using ultrasound. For example, proposed methods include a method of arranging two transducers for transmitting and receiving to face each other and obtaining a sound speed value in a subject based on a distance between the transducers and a propagation time of ultrasound, and a method of using two pairs of transducers arranged at a predetermined interval as the transducers for transmitting and receiving respectively and obtaining a propagation speed based on a propagation time of ultrasound between the transducers, wave transmitting angles and wave receiving angles, and the interval between the transducers of each pair. Japanese Patent Application Laid-Open No. 5-95946 discloses a measurement method of a local sound speed value described below. In Japanese Patent Application Laid-Open No. 5-95946, a wave transmitting transducer transmits ultrasound into a subject while changing an emission angle, a wave receiving transducer receives the ultrasound while changing an incident angle, and times elapsing between transmitting and receiving of the ultrasound are all stored in a memory. Then, assumed sound speed distribution is set, and a time elapsing between transmitting and receiving of the ultrasound for each emission angle and each incident angle is calculated on the basis of the sound speed distribution. Then, the assumed sound speed distribution is corrected so as to minimize a difference between a calculated value and an actual measured value of the time, and a sound speed value in the subject is obtained from sound speed distribution obtained finally. SUMMARY OF THE INVENTION A sound speed value V in a subject OBJ1 including a medium with a constant sound speed value can be calculated as described below. As shown in FIG. 14A, a time T elapsing between a time ultrasound is reflected at a reflection point (region) X1ROI and a time the ultrasound is received by an element 302A0 located immediately below the reflection point X1ROI is T=L/V, where L is a distance between the reflection point X1ROI in the subject OBJ1 and an ultrasound probe 300A. When T+ΔT is a time elapsing between a time the ultrasound is reflected at the reflection point X1ROI and a time the ultrasound is received by an element 302Ai located at a distance X from the element 302A0 in an X direction (an arrangement direction of elements 302A), a delay time ΔT between the element 302A0 and the element 302Ai is expressed by the following expression (1): ΔT=ΔL/V(where ΔL=√{square root over (L2+X2)}−L) (1) Thus, time [2T, 2T+ΔT] elapsing between a time the ultrasound is transmitted and reflected at the reflection point X1ROI the time T later and a time the ultrasound is received by the elements can be measured to uniquely obtain the distance L between the ultrasound probe 300A and the reflection point X1ROI and the speed V. When the ultrasound from the reflection point X1ROI can be clearly identified, L and V can be determined from the times measured at two different elements having a known positional relationship. However, ultrasound detection signals output from the elements 302A are generally generated by interference of signals from countless reflection points, and it is difficult to identify a signal from a specific reflection point. Thus, actually, the distance L to the reflection point X1ROI, the delay time ΔT, and the sound speed value V are uniquely obtained based on a space frequency, sharpness, and contrast of a reconstructed image in a region of interest near the reflection point X1ROI. As described above, when the sound speed in the subject is constant, the sound speed value can be obtained, while when the sound speed in the subject is not constant as in a subject OBJ2 shown in FIG. 14B, it is difficult to obtain a distance L between the ultrasound probe 300A and a reflection point (region) X2ROI and sound speed values V and V′ by the above described method. The present invention is achieved in view of these circumstances, and has an object to provide an ultrasound diagnostic apparatus and an ultrasound diagnostic method which enable to calculate a sound speed value (local sound speed value) in any region to be diagnosed in a subject with high accuracy. To solve the above described problems, an ultrasound diagnostic apparatus of a first aspect of the present invention includes: an ultrasound probe including a plurality of ultrasound transducers for transmitting ultrasound to a subject and receiving the ultrasound reflected by the subject to output an ultrasound detection signal; an optimum sound speed value determination device for determining optimum sound speed values at a lattice point set in a shallower region than a region of interest in the subject and in the region of interest on the basis of the ultrasound detection signal; a first arithmetical operation device for arithmetically operating a received wave received from the region of interest when the ultrasound is transmitted to the region of interest on the basis of the optimum sound speed value in the region of interest; a second arithmetical operation device for setting an assumed sound speed in the region of interest, obtaining a received wave received from each lattice point when the ultrasound is transmitted to the region of interest on the basis of the assumed sound speed and the optimum sound speed value at the lattice point, and synthesizing received waves at lattice points to obtain a resultant received wave; and a local sound speed value determination device for making a determination of a local sound speed value in the region of interest on the basis of the received wave and the resultant received wave. According to the first aspect, the local sound speed value in the subject can be determined with high accuracy using an amplitude image obtained in generation of a B (Brightness) mode image, RF (Radio Frequency) data, or data on the received waves received by the ultrasound transducer elements. The local sound speed value obtained as described above can be used to detect a lesion in the subject with higher accuracy. In the first aspect, a configuration exclusively for transmitting ultrasound and a configuration exclusively for receiving ultrasound are not required to be arranged separately, in order to measure the local sound speed value. The ultrasound diagnostic apparatus of a second aspect of the present invention is such that the local sound speed value determination device determines the assumed sound speed used for arithmetical operation of a received wave at which a delta between the received wave and the resultant received wave is minimized, as a local sound speed value in the region of interest, in the first aspect. An ultrasound diagnostic apparatus of a third aspect of the present invention includes: an ultrasound probe including a plurality of ultrasound transducers for transmitting ultrasound to a subject and receiving the ultrasound reflected by the subject to output an ultrasound detection signal; an optimum sound speed value determination device for determining an optimum sound speed value in a region between a plurality of lattice points set in a shallower region than a region of interest in the subject and the ultrasound probe on the basis of the ultrasound detection signal; a resultant received wave obtaining device for obtaining received waves received from the plurality of lattice points, and synthesizing the obtained received waves to obtain a first resultant received wave; an arithmetical operation device for setting an assumed sound speed in the region of interest, tracing an acoustic ray of ultrasound transmitted from the ultrasound probe to the region of interest on the basis of the assumed sound speed and optimum sound speed values at the plurality of lattice points, obtaining received waves reflected at the region of interest and reaching the plurality of ultrasound transducers through the plurality of lattice points, and synthesizing the obtained received waves to obtain a second resultant received wave; and a local sound speed value determination device for making a determination of a local sound speed value in the region of interest on the basis of the first and the second resultant received waves. According to the third aspect, the acoustic ray tracing of the ultrasound reflected at the region of interest in the subject is performed, and the local sound speed value in the region of interest is determined using the received waves directly reaching the ultrasound transducers (elements) of the ultrasound probe, thereby allowing the local sound speed value to be accurately calculated with fewer processes. The ultrasound diagnostic apparatus of a fourth aspect of the present invention is such that the local sound speed value determination device determines an assumed sound speed used for arithmetical operation of a received wave at which a delta between the first resultant received wave and the second resultant received wave is minimized, as a local sound speed value in the region of interest, in the third aspect. An ultrasound diagnostic apparatus of a fifth aspect of the present invention includes: an ultrasound probe including a plurality of ultrasound transducers for transmitting ultrasound to a subject and receiving the ultrasound reflected by the subject to output an ultrasound detection signal; an arithmetical operation device for setting an assumed sound speed in a certain region of interest in the subject, obtaining received waves received from a plurality of lattice points set in a shallower region than the region of interest when ultrasound is transmitted from the ultrasound probe to the region of interest on the basis of the assumed sound speed, and synthesizing the received waves from the lattice points to obtain a resultant received wave; a sound speed determination index arithmetical operation device for obtaining a delay from the received waves from the lattice points to generate an image in the region of interest on the basis of the delay, and obtaining a sound speed determination index on the basis of the image; and a local sound speed value determination device for making a determination of a local sound speed value in the region of interest on the basis of the sound speed determination index, and repeating the determination of the local sound speed value in order from a lattice point in a shallow region to determine the local sound speed value in the subject. According to the fifth aspect, the local sound speed value is determined in order from a shallow layer in the subject, thereby allowing the local sound speed value in the subject to be determined with high accuracy. The ultrasound diagnostic apparatus of a sixth aspect of the present invention is such that the sound speed determination index arithmetical operation device obtains the sound speed determination index on the basis of at least one of contrast of the image, a space frequency of the image in a scanning direction, and dispersion of the space frequency, in the fifth aspect. An ultrasound diagnostic apparatus of a seventh aspect of the present invention includes: an ultrasound probe including a plurality of ultrasound transducers for transmitting ultrasound to a subject and receiving the ultrasound reflected by the subject to output an ultrasound detection signal; an arithmetical operation device for setting an assumed sound speed in a certain region of interest in the subject, tracing an acoustic ray of the ultrasound transmitted from the ultrasound probe to the region of interest, and obtaining received waves reflected at the region of interest and reaching the plurality of ultrasound transducers through the plurality of lattice points set in a shallower region than the region of interest on the basis of the assumed sound speed; a sound speed determination index arithmetical operation device for obtaining a delay from the received waves from the plurality of lattice points to generate an image in the region of interest on the basis of the delay, and obtaining a sound speed determination index on the basis of the image; and a local sound speed value determination device for making a determination of a local sound speed value in the region of interest on the basis of the sound speed determination index, and repeating the determination of the local sound speed value in order from a lattice point in a shallow region to determine the local sound speed value in the subject. According to the seventh aspect, acoustic ray tracing is repeatedly conducted in order from the shallow layer in the subject to determine the local sound speed value, thereby allowing the local sound speed value in the subject to be determined with high accuracy. The ultrasound diagnostic apparatus of an eighth aspect of the present invention further includes a lattice point setting device for setting a lattice point on a boundary between regions with a substantially uniform sound speed in the subject, in any one of the first to seventh aspects. The ultrasound diagnostic apparatus of a ninth aspect of the present invention further includes an amplitude image generation device (100, 502) for performing at least one of a transmitting focus process and a receiving focus process on the basis of the determination result of the local sound speed value in the subject to generate an amplitude image indicating an amplitude of the received wave with brightness of a point, in any one of the first to eighth aspects. The ultrasound diagnostic apparatus of a tenth aspect of the present invention further includes a display device for displaying the determination result of the local sound speed value, in any one of the first to eighth aspects. The ultrasound diagnostic apparatus of an eleventh aspect of the present invention further includes an amplitude image generation device for generating an amplitude image indicating an amplitude of the received wave with brightness of a point on the basis of the ultrasound detection signal, wherein the display device displays the determination result of the local sound speed value so as to be superimposed on the amplitude image or placed with the amplitude image side by side, in the tenth aspect. The ultrasound diagnostic apparatus of a twelfth aspect of the present invention further includes an amplitude image generation device for generating an amplitude image indicating an amplitude of the ultrasonic echo with brightness of a point on the basis of the ultrasound detection signal, wherein the display device displays the determination result of the local sound speed value by changing brightness or color of the amplitude image, in the tenth aspect. The ultrasound diagnostic apparatus of a thirteenth aspect of the present invention further includes a display mode switching device for switching a display mode between a first display mode in which the amplitude image is displayed alone and a second display mode in which the determination result of the local sound speed value in the subject is displayed, in the eleventh or twelfth aspect. According to the tenth to thirteenth aspects, a device for an operator to observe the determination result of the local sound speed value can be provided in addition to the above described advantages. An ultrasound diagnostic method of a fourteenth aspect of the present invention includes: an optimum sound speed value determination step of determining optimum sound speed values at a lattice point set in a shallower region than a region of interest in a subject and in the region of interest on the basis of an ultrasound detection signal obtained by an ultrasound probe, the ultrasound probe including a plurality of ultrasound transducers for transmitting ultrasound to the subject and receiving the ultrasound reflected by the subject to output the ultrasound detection signal; a first arithmetical operation step of arithmetically operating a received wave received from the region of interest when the ultrasound is transmitted to the region of interest on the basis of the optimum sound speed value in the region of interest; a second arithmetical operation step of setting an assumed sound speed in the region of interest, obtaining a received wave received from each lattice point when the ultrasound is transmitted to the region of interest on the basis of the assumed sound speed and the optimum sound speed value at the lattice point, and synthesizing received waves at lattice points to obtain a resultant received wave; and a local sound speed value determination step of making a determination of a local sound speed value in the region of interest on the basis of the received wave and the resultant received wave. The ultrasound diagnostic method of a fifteenth aspect of the present invention is such that the assumed sound speed used for arithmetical operation of a received wave at which a delta between the received wave and the resultant received wave is minimized is determined as a local sound speed value in the region of interest in the local sound speed value determination step in the fourteenth aspect. An ultrasound diagnostic method of a sixteenth aspect of the present invention includes: an optimum sound speed value determination step of determining an optimum sound speed value in a region between a plurality of lattice points set in a shallower region than a region of interest in a subject and an ultrasound probe on the basis of an ultrasound detection signal obtained by the ultrasound probe, the ultrasound probe including a plurality of ultrasound transducers for transmitting ultrasound to the subject and receiving the ultrasound reflected by the subject to output the ultrasound detection signal; a resultant received wave obtaining step of obtaining received waves received from the plurality of lattice points, and synthesizing the obtained received waves to obtain a first resultant received wave; an arithmetical operation step of setting an assumed sound speed in the region of interest, tracing an acoustic ray of ultrasound transmitted from the ultrasound probe to the region of interest on the basis of the assumed sound speed and optimum sound speed values at the plurality of lattice points, obtaining received waves reflected at the region of interest and reaching the plurality of ultrasound transducers through the plurality of lattice points, and synthesizing the obtained received waves to obtain a second resultant received wave; and a local sound speed value determination step of making a determination of a local sound speed value in the region of interest on the basis of the first and the second resultant received waves. The ultrasound diagnostic method of a seventeenth aspect of the present invention is such that the assumed sound speed used for arithmetical operation of a received wave at which a delta between the first resultant received wave and the second resultant received wave is minimized is determined as a local sound speed value in the region of interest in the local sound speed value determination step in the sixteenth aspect. An ultrasound diagnostic method of an eighteenth aspect of the present invention includes: an arithmetical operation step of setting an assumed sound speed in a certain region of interest in a subject, obtaining received waves received from a plurality of lattice points set in a shallower region than the region of interest when ultrasound is transmitted from an ultrasound probe to the region of interest on the basis of the assumed sound speed, and synthesizing the received waves from the lattice points to obtain a resultant received wave; a sound speed determination index arithmetical operation step of obtaining a delay from the received waves from the lattice points to generate an image in the region of interest on the basis of the delay, and obtaining a sound speed determination index on the basis of the image; and a local sound speed value determination step of making a determination of the local sound speed value in the region of interest on the basis of the sound speed determination index, and repeating the determination of the local sound speed value in order from a lattice point in a shallow region to determine the local sound speed value in the subject. The ultrasound diagnostic method of a nineteenth aspect of the present invention is such that the sound speed determination index is obtained on the basis of at least one of contrast of the image, a space frequency of the image in a scanning direction, and dispersion of the space frequency in the sound speed determination index arithmetical operation step in the eighteenth aspect. An ultrasound diagnostic method of a twentieth aspect of the present invention includes: an arithmetical operation step of setting an assumed sound speed in a certain region of interest in a subject, tracing an acoustic ray of ultrasound transmitted from a ultrasound probe to the region of interest, the ultrasound probe including a plurality of ultrasound transducers for transmitting ultrasound to the subject and receiving the ultrasound reflected by the subject to output an ultrasound detection signal, and obtaining received waves reflected at the region of interest and reaching the plurality of ultrasound transducers through the plurality of lattice points set in a shallower region than the region of interest on the basis of the assumed sound speed; a sound speed determination index arithmetical operation step of obtaining a delay from the received waves from the plurality of lattice points to generate an image in the region of interest on the basis of the delay, and obtaining a sound speed determination index on the basis of the image; and a local sound speed value determination step of making a determination of the local sound speed value in the region of interest on the basis of the sound speed determination index, and repeating the determination of the local sound speed value in order from a lattice point in a shallow region to determine the local sound speed value in the subject. The ultrasound diagnostic method of a twenty-first aspect of the present invention further includes a lattice point setting step of setting a lattice point on a boundary between regions with a substantially uniform sound speed in the subject, in any one of the fourteenth to twentieth aspects. The ultrasound diagnostic method of a twenty-second aspect of the present invention further includes an amplitude image generation step of performing at least one of a transmitting focus process and a receiving focus process on the basis of a determination result of the local sound speed value in the subject to generate an amplitude image indicating an amplitude of the received wave with brightness of a point, in any one of the fourteenth to twenty-first aspects. The ultrasound diagnostic method of a twenty-third aspect of the present invention further includes a display step of displaying the determination result of the local sound speed value on a display device, in any one of the fourteenth to twenty-first aspects. The ultrasound diagnostic method of a twenty-fourth aspect of the present invention further includes: an amplitude image generation step of generating an amplitude image indicating an amplitude of the received wave with brightness of a point on the basis of the ultrasound detection signal; and a step of displaying the determination result of the local sound speed value on the display device so as to be superimposed on the amplitude image or placed with the amplitude image side by side, in the twenty-third aspects. The ultrasound diagnostic method of a twenty-fifth aspect of the present invention further includes: an amplitude image generation step of generating an amplitude image indicating an amplitude of the ultrasonic echo with brightness of a point on the basis of the ultrasound detection signal; and a step of displaying the determination result of the local sound speed value on the display device by changing brightness or color of the amplitude image, in the twenty-third aspect. The ultrasound diagnostic method of a twenty-sixth aspect of the present invention further includes: a display mode switching step of receiving an instruction input from an operator by a display mode switching device to switch a display mode between a first display mode in which the amplitude image is displayed alone and a second display mode in which the determination result of the local sound speed value in the subject is displayed, and switching the display mode, in the twenty-fourth or twenty-fifth aspect. According to the present invention, the local sound speed value in the subject can be determined with high accuracy using an amplitude image obtained in generation of a B mode image, RF data, or data on the received waves received by the ultrasound transducer elements. The local sound speed value obtained as described above can be used to detect a lesion in the subject with higher accuracy. A configuration exclusively for transmitting ultrasound and a configuration exclusively for receiving ultrasound are not required to be arranged separately, in order to measure the local sound speed value. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an ultrasound diagnostic apparatus according to a first embodiment of the present invention; FIGS. 2A and 2B schematically show an arithmetical operation process of a local sound speed value according to the first embodiment of the present invention; FIG. 3 is a flowchart showing the arithmetical operation process of the local sound speed value according to the first embodiment of the present invention; FIG. 4 is a flowchart showing the arithmetical operation process of the local sound speed value according to the first embodiment of the present invention; FIG. 5 schematically shows an arithmetical operation process of a local sound speed value according to a second embodiment of the present invention; FIG. 6 is a flowchart showing the arithmetical operation process of the local sound speed value according to the second embodiment of the present invention; FIG. 7 is a flowchart showing the arithmetical operation process of the local sound speed value according to the second embodiment of the present invention; FIG. 8 schematically shows an arithmetical operation procedure of a local sound speed value in a subject according to a third embodiment of the present invention; FIG. 9 is a flowchart showing an arithmetical operation process of the local sound speed value according to the third embodiment of the present invention; FIG. 10 is a flowchart showing a calculation process of a receiving time of a received wave by acoustic ray tracing according to the third embodiment of the present invention; FIG. 11 schematically shows an arithmetical operation procedure of a local sound speed value in a subject according to a fourth embodiment of the present invention; FIG. 12 is a flowchart showing a calculation process of a receiving time of a received wave by acoustic ray tracing according to the fourth embodiment of the present invention; FIG. 13 is a flowchart showing an arithmetical operation process of the local sound speed value according to a fifth embodiment of the present invention; and FIGS. 14A and 14B schematically show an arithmetical operation process of a local sound speed value. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, preferred embodiments of an ultrasound diagnostic apparatus and an ultrasound diagnostic method according to the present invention will be described with reference to the accompanying drawings. First Embodiment FIG. 1 is a block diagram of an ultrasound diagnostic apparatus according to a first embodiment of the present invention. An ultrasound diagnostic apparatus 10 shown in FIG. 1 is an apparatus that transmits an ultrasound beam from an ultrasound probe 300 to a subject OBJ, receives the ultrasound beam (ultrasonic echo) reflected by the subject OBJ, and generates and displays an ultrasound image from a detection signal of the ultrasonic echo. A CPU (Central Processing Unit) 100 controls each block in the ultrasound diagnostic apparatus 10 according to an operation input from an operation input unit 200. The operation input unit 200 is an input device that receives an operation input by an operator, and includes a console 202 and a pointing device 204. The console 202 includes a keyboard that receives an input of character information (for example, patient information), a display mode switch button for switching a display mode between a mode in which an amplitude image (B (Brightness) mode image) is displayed alone and a mode in which a determination result of a local sound speed value is displayed, a freeze button for giving instruction to switch between a live mode and a freeze mode, a cine memory reproduction button for giving instruction to reproduce an image based on data stored in a cine memory, and an analysis and measurement button for giving instruction to analyze and measure an ultrasound image. The pointing device 204 is a device that receives an input of designation of a region on a screen of a display unit 104, for example, a trackball or a mouse. A touch panel may be used as the pointing device 204. A storage unit 102 is a storage device that stores a control program for the CPU 100 to control each block in the ultrasound diagnostic apparatus 10, for example, a hard disk or a semiconductor memory. The display unit 104 is, for example, a CRT (Cathode Ray Tube) display or a liquid crystal display, and displays an ultrasound image (moving image and still image) and various setting screens. The ultrasound probe 300 is a probe to be abutted against the subject OBJ for use, and includes a plurality of ultrasound transducers 302 that constitute a one-dimensional or two-dimensional transducer array. The ultrasound transducer 302 transmits an ultrasound beam to the subject OBJ on the basis of a drive signal applied from a transmission circuit 402, and receives an ultrasonic echo reflected from the subject OBJ to output a detection signal. The ultrasound transducer 302 includes a transducer including electrodes formed at opposite ends of a piezoelectric material (piezoelectric body). As the piezoelectric body included in the transducer, for example, piezoelectric ceramic such as PZT (Pb (lead) zirconate titanate) or a polymeric piezoelectric element such as PVDF (polyvinylidene difluoride) can be used. When an electric signal is sent to the electrode of the transducer to apply a voltage, the piezoelectric body expands or contracts, and the expansion and contraction of the piezoelectric body causes each transducer to generate ultrasound. For example, when a pulse-like electric signal is sent to the electrode of the transducer, pulse-like ultrasound is generated, and when a continuous wave electric signal is sent to the electrode of the transducer, continuous wave ultrasound is generated. Then, ultrasound generated from each transducer is synthesized to form an ultrasound beam. When each transducer receives ultrasound, the piezoelectric body in each transducer expands and contracts to generate an electric signal. The electric signal generated from each transducer is output to a reception circuit 404 as an ultrasound detection signal. As the ultrasound transducers 302, a plurality of kinds of elements of different ultrasound conversion types may be used. For example, the transducer including the piezoelectric body is used as an element that transmits ultrasound, and an optical detection type ultrasound transducer may be used as an element that receives ultrasound. The optical detection type ultrasound transducer converts an ultrasound signal into an optical signal for detection, and is, for example, a Fabry-Perot resonator or a fiber Bragg grating. Next, an ultrasound diagnosis process in the live mode will be described. The live mode is a mode in which an ultrasound image (moving image) obtained by abutting the ultrasound probe 300 against the subject OBJ to transmit and receive ultrasound is displayed, analyzed and measured. When the ultrasound probe 300 is abutted against the subject OBJ, and ultrasound diagnosis is started by an instruction input from the operation input unit 200, the CPU 100 outputs an control signal to a transmission and reception unit 400, and starts transmitting an ultrasound beam to the subject OBJ and receiving an ultrasonic echo from the subject OBJ. The CPU 100 sets a transmitting direction of the ultrasound beam and a receiving direction of the ultrasonic echo for each ultrasound transducer 302. Further, the CPU 100 selects a transmission delay pattern according to the transmitting direction of the ultrasound beam, and selects a reception delay pattern according to the receiving direction of the ultrasonic echo. The transmission delay pattern is pattern data of a delay time provided to a drive signal for forming an ultrasound beam in a desired direction by ultrasound transmitted from the plurality of ultrasound transducers 302. The reception delay pattern is pattern data of a delay time provided to a detection signal for extracting an ultrasonic echo from a desired direction by ultrasound received by the plurality of ultrasound transducers 302. The transmission delay pattern and the reception delay pattern are previously stored in the storage unit 102. The CPU 100 selects a transmission delay pattern and a reception delay pattern from the patterns stored in the storage unit 102, and outputs a control signal to the transmission and reception unit 400 to perform control to transmit and receive ultrasound according to the selected transmission delay pattern and reception delay pattern. The transmission circuit 402 generates a drive signal according to a control signal from the CPU 100, and applies the drive signal to the ultrasound transducer 302. At this time, the transmission circuit 402 delays the drive signal applied to each ultrasound transducer 302 on the basis of the transmission delay pattern selected by the CPU 100. The transmission circuit 402 adjusts (delays) timing of applying the drive signal to each ultrasound transducer 302 so that ultrasound transmitted from the plurality of ultrasound transducers 302 forms an ultrasound beam. The timing of applying the drive signal may be adjusted so that ultrasound transmitted at one time from the plurality of ultrasound transducers 302 reaches the entire imaging region of the subject OBJ. The reception circuit 404 receives and amplifies an ultrasound detection signal output from each ultrasound transducer 302. As described above, because of different distances between the respective ultrasound transducers 302 and an ultrasound reflection source in the subject OBJ, reflected waves reach the respective ultrasound transducers 302 at different times. The reception circuit 404 includes a delay circuit, and delays each detection signal by an amount corresponding to a difference (delay time) between reaching times of the reflected waves according to a sound speed (hereinafter referred to as an assumed sound speed) set on the basis of the reception delay pattern selected by the CPU 100 or distribution of the sound speed. Then, the reception circuit 404 performs matching addition of the detection signals delayed by the delay time to perform a receiving focus process. When there is a different ultrasound reflection source in a different position from an ultrasound reflection source XROI, an ultrasound detection signal from the different ultrasound reflection source reaches at a different time. Thus, the addition by an addition circuit of the reception circuit 404 causes phases of ultrasound detection signals from different ultrasound reflection sources to cancel each other out. Thus, the received signal from the ultrasound reflection source XROI is maximized and comes into focus. The receiving focus process forms an acoustic ray signal (hereinafter referred to as an RF (Radio Frequency) signal) with narrowed focus of an ultrasonic echo. An A/D converter 406 converts an analog RF signal output from the reception circuit 404 into a digital RF signal (hereinafter referred to as RF data). The RF data includes phase information of a received wave (carrier wave). The RF data output from the A/D converter 406 is input to a signal processing unit 502 and a cine memory 602. The cine memory 602 successively stores the RF data input from the A/D converter 406. The cine memory 602 stores information on a frame rate (for example, a depth of a reflection position of ultrasound, a density of scanning lines, a parameter indicating a width of field of view) input from the CPU 100 in association with the RF data. The signal processing unit 502 corrects the RF data in attenuation due to a distance according to a depth of a reflection position of ultrasound by STC (Sensitivity Time gain Control), and then performs an envelope detection process of the RF data to generate a B mode image data (image data indicating an amplitude of an ultrasonic echo with brightness of a point). The B mode image data generated by the signal processing unit 502 is obtained by a different scanning method from a normal television signal scanning method. Thus, a DSC (Digital Scan Converter) 504 converts (raster conversion) the B mode image data into normal image data (for example, image data by the television signal scanning method (NTSC (National Television System Committee in the United States) method)). An image processing unit 506 performs various necessary image processes (for example, gradation processing) on image data input from the DSC 504. An image memory 508 stores image data input from the image processing unit 506. A D/A converter 510 converts image data read from the image memory 508 into an analog image signal, and outputs the signal to the display unit 104. Thus, the ultrasound image (moving image) taken by the ultrasound probe 300 is displayed on the display unit 104. In the embodiment, the detection signal subjected to the receiving focus process in the reception circuit 404 is the RF signal, but a detection signal that is not subjected to the receiving focus process may be an RF signal. In this case, the plurality of ultrasound detection signals output from the plurality of ultrasound transducers 302 are amplified by the reception circuit 404, and the amplified detection signals, that is, the RF signals are A/D converted by the A/D converter 406 to generate RF data. Then, the RF data is supplied to the signal processing unit 502, and stored in the cine memory 602. The receiving focus process is performed by the signal processing unit 502 in digital form. Next, a cine memory reproduction mode will be described. The cine memory reproduction mode is a mode in which an ultrasound diagnosis image is displayed, analyzed and measured on the basis of the RF data stored in the cine memory 602. When the cine memory reproduction button on the console 202 is pushed, the CPU 100 switches an operation mode of the ultrasound diagnostic apparatus 10 to the cine memory reproduction mode. In the cine memory reproduction mode, the CPU 100 instructs a cine memory reproduction unit 604 to reproduce RF data designated by an operation input by an operator. The cine memory reproduction unit 604 reads the RF data from the cine memory 602 according to the instruction from the CPU 100, and transmits the read RF data to the signal processing unit 502 in the image signal generation unit 500. The RF data transmitted from the cine memory 602 is subjected to predetermined processes (the same processes as in the live mode) by the signal processing unit 502, the DSC 504, and the image processing unit 506 and converted into image data, and then output via an image memory 508 and the D/A converter 510 to the display unit 104. Thus, the ultrasound image (moving image or still image) based on the RF data stored in the cine memory 602 is displayed on the display unit 104. When the freeze button on the console 202 is pushed while the ultrasound image (moving image) is displayed in the live mode or the cine memory reproduction mode, the ultrasound image displayed when the freeze button is pushed is displayed as a still image on the display unit 104. This allows the operator to display and observe a still image of a region of interest (ROI). When the measurement button on the console 202 is pushed, analysis and measurement designated by the operation input by the operator are performed. When the measurement button is pushed in each operation mode, a data analysis and measurement unit 106 obtains RF data before image processing from the A/D converter 406 or the cine memory 602, and uses the RF data to perform analysis and measurement designated by the operator (for example, distortion analysis of a tissue part (diagnosis of hardness), measurement of blood flow, measurement of movement of the tissue part, or measurement of IMT (Intima-Media Thickness) value). Results of analysis and measurement by the data analysis and measurement unit 106 are output to the DSC 504 in the image signal generation unit 500. The DSC 504 inserts the results of the analysis and measurement by the data analysis and measurement unit 106 into image data of the ultrasound image and outputs the data to the display unit 104. Thus, the ultrasound image and the results of the analysis and measurement are displayed on the display unit 104. When the display mode switching button is pushed, the display mode is switched between a mode in which the B mode image is displayed alone, a mode in which the determination result of the local sound speed value is displayed so as to be superimposed on the B mode image (for example, display with changing color or brightness according to the local sound speed value or display with a line connecting points with the same local sound speed value), and a mode in which the B mode image and the image of the determination result of the local sound speed value are placed side by side. This allows the operator to observe the determination result of the local sound speed value to find, for example, a lesion. A B mode image obtained by performing at least one of the transmitting focus process and the receiving focus process may be displayed on the display unit 104 on the basis of the determination result of the local sound speed value. [Arithmetical Operation Process of Local Sound Speed Value] FIGS. 2A and 2B schematically show an arithmetical operation process of the local sound speed value according to the first embodiment of the present invention. As shown in FIG. 2B, a representative lattice point in the region of interest ROI in the subject OBJ is determined as XROI, and lattice points arranged at regular intervals in X direction in a position shallower than the lattice point XROI (that is, closer to the ultrasound transducer 302) are determined as A1, A2, . . . , and it is assumed that a sound speed at least between the lattice point XROI and each of the lattice points A1, A2, . . . is constant. In the embodiment, T and delay times ΔT of received waves (WA1, WA2, . . . , respectively) from the lattice points A1, A2, . . . are regarded as known, and a local sound speed value at the lattice point XROI is obtained from a positional relationship between the lattice point XROI and the lattice points A1, A2, . . . . Specifically, matching is used between an assumed received wave WX from the lattice point XROI and a received wave WSUM of an assumed synthesis of the received waves from the lattice points A1, A2, . . . by Huygens' principle. A range (a region where the lattice points A1, A2, . . . are arranged) and the number of lattice points A1, A2, . . . used in an arithmetical operation for obtaining the local sound speed value at the lattice point XROI are previously determined. A wide range of lattice points used for the arithmetical operation of the local sound speed value causes a large error in the local sound speed value, and a narrow range causes a large error from the assumed received wave, and thus the range of lattice points is determined by a trade-off therebetween. The interval between the lattice points A1, A2, . . . in the X direction is determined by a trade-off between resolution and processing time. The interval between the lattice points A1, A2, . . . in the X direction is, for example, nearly 1 mm to 1 cm. A narrow interval between the lattice points A1, A2, . . . and XROI in the Y direction causes a large error in error calculation, and a wide interval between the lattice points A1, A2, . . . and XROI in the Y direction causes a large error in the local sound speed value. The interval between the lattice points A1, A2, . . . and XROI in the Y direction is determined on the basis of setting of image resolution of an ultrasound image. The interval between the lattice points A1, A2, . . . and XROI in the Y direction is, for example, nearly 1 cm. The wide interval between the lattice points A1, A2, . . . makes difficult an arithmetical operation (described later) of a resultant wave, and thus fine lattice points may be generated by interpolation. Now, the arithmetical operation process of the local sound speed value according to the embodiment will be described with reference to flowcharts in FIGS. 3 and 4. In the embodiment, first, as shown in FIG. 3, optimum sound speed values at all lattice points set in the subject OBJ are previously determined (step S10). The optimum sound speed value is a sound speed value with the highest contrast and sharpness of an image, and does not always match the actual local sound speed value at each lattice. As a determination method of the optimum sound speed value in step S10, for example, a method of determining an optimum sound speed value from contrast of the image, a space frequency of the image in a scanning direction, and dispersion of the space frequency (for example, Japanese Patent Application Laid-Open No. 8-317926) may be applied. Next, a local sound speed value at each lattice point is determined on the basis of the optimum sound speed value at each lattice point (step S12). A method of determining the local sound speed value on the basis of the optimum sound speed value will be described. First, as shown in FIG. 4, a waveform of an assumed received wave WX with the lattice point XROI as a reflection point is calculated on the basis of the optimum sound speed value at the lattice point XROI (step S20). Then, an initial value of an assumed sound speed at the lattice point XROI is set (step S22). Then, the assumed sound speed is changed by one step (step S24), and an assumed resultant received wave WSUM is calculated (step S26). When the local sound speed value at the lattice point XROI is assumed as V, times that elapses after the ultrasound propagating from the lattice point XROI before reaching the lattice points A1, A2, . . . are XROIA1/V, XROIA2/V, . . . respectively. The XROIA1, XROIA2, . . . are distances between the lattice points A1, A2, . . . , respectively, and the lattice point XROI. The optimum sound speed values at the lattice points A1, A2, . . . are known by step S10, and thus received waves from the lattice points A1, A2, . . . can be previously obtained. Thus, reflected waves (ultrasonic echo) generated with delays XROIA1/V, XROIA2/V, . . . from the lattice points A1, A2, . . . can be synthesized to obtain the assumed resultant received wave WSUM. Actually, the above described processing is performed on element data (RF signal), and thus times (T1, T2, . . . respectively) that elapses after the ultrasound propagating from the lattice point XROI before reaching the lattice points A1, A2, . . . are expressed by the following expression (2): T1=√{square root over ((XA1/V)2+(Δt/2)2)}{square root over ((XA1/V)2+(Δt/2)2)}, T2=√{square root over ((XA2/V)2+(Δt/2)2)}{square root over ((XA2/V)2+(Δt/2)2)}, T3= .
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