9. A speech synthesis method comprising: dividing a phoneme string corresponding to target speech into a plurality of segments to generate a first segment sequence; generating a plurality of first speech unit strings corresponding to the first segment sequence by combining a plurality of speech units based on the first segment sequence and selecting one speech unit string from said plurality of first speech unit strings; and concatenating a plurality of speech units included in the selected speech unit string to generate synthetic speech, the generating/selecting including performing repeatedly a first processing and a second processing, the first processing generating, based on maximum W, wherein W is a predetermined value, second speech unit strings corresponding to a second segment sequence as a partial sequence of the first segment sequence, a plurality of third speech unit strings corresponding to a third segment sequence as a partial sequence obtained by adding a segment to the second segment sequence, and the second processing selecting maximum W third speech unit strings from said plurality of third speech unit strings, calculating a total cost of each of said plurality of third speech unit strings, calculating a penalty coefficient corresponding to the total cost for each of said plurality of third speech unit strings based on a restriction concerning quickness of speech unit data acquisition, wherein the penalty coefficient depending on extent in which the restriction is approached, and calculating a evaluation value of each of said plurality of third speech unit strings by correcting the total cost with the penalty coefficient, wherein the second processing including selecting the maximum W third speech unit strings from said plurality of third speech unit strings based on the evaluation value of each of said plurality of third speech unit strings.
9. A speech synthesis method comprising: dividing a phoneme string corresponding to target speech into a plurality of segments to generate a first segment sequence; generating a plurality of first speech unit strings corresponding to the first segment sequence by combining a plurality of speech units based on the first segment sequence and selecting one speech unit string from said plurality of first speech unit strings; and concatenating a plurality of speech units included in the selected speech unit string to generate synthetic speech, the generating/selecting including performing repeatedly a first processing and a second processing, the first processing generating, based on maximum W, wherein W is a predetermined value, second speech unit strings corresponding to a second segment sequence as a partial sequence of the first segment sequence, a plurality of third speech unit strings corresponding to a third segment sequence as a partial sequence obtained by adding a segment to the second segment sequence, and the second processing selecting maximum W third speech unit strings from said plurality of third speech unit strings, calculating a total cost of each of said plurality of third speech unit strings, calculating a penalty coefficient corresponding to the total cost for each of said plurality of third speech unit strings based on a restriction concerning quickness of speech unit data acquisition, wherein the penalty coefficient depending on extent in which the restriction is approached, and calculating a evaluation value of each of said plurality of third speech unit strings by correcting the total cost with the penalty coefficient, wherein the second processing including selecting the maximum W third speech unit strings from said plurality of third speech unit strings based on the evaluation value of each of said plurality of third speech unit strings. 10. The method according to claim 9 , further comprising: preparing in advance a first storage unit including a plurality of storage mediums with different data acquisition speeds, which store a plurality of speech units, respectively; preparing in advance a second storage unit configured to store information indicating in which one of said plurality of storage mediums each of the speech units is stored; and acquiring the plurality of speech units from the first storage unit in accordance with the information before concatenating the plurality of speech units, and wherein the calculating the penalty coefficient including calculating the penalty coefficient for each of said plurality of third speech unit strings based on a restriction concerning quickness of data acquisition which is to be satisfied when the speech units included in the first speech unit string are acquired from the first storage unit by the concatenation unit and a statistic determined depending on which one of said plurality of storage mediums each of all speech units included in the third speech unit string is stored in.