1. A system for diagnosing a state of a construction machine in accordance with at least parameter data acquired by sensors, the system comprising: a diagnosis device for diagnosing the construction machine; a diagnostic knowledge storage device for storage of diagnostic knowledge including a diagnosing technique used for the diagnosis; and a diagnostic data storage device for storage of diagnostic data including the sensor-acquired parameter data, the diagnostic data being used for the diagnosis; wherein the diagnosis device includes: an input and output section for data input and output with respect to outside; a diagnostic data acquisition section for acquiring, on the basis of the data input to the input and output section, diagnostic data including appropriate parameter data, from the diagnostic data storage device; a data characteristics acquisition section for acquiring data characteristics of the diagnostic data acquired by the diagnostic data acquisition section, from the acquired diagnostic data, the data characteristics being inclusive of at least parameter characteristics which denote characteristics of the parameter data; a diagnostic knowledge acquisition section for acquiring, from the diagnostic knowledge storage device, any diagnosing techniques fitting the data characteristics acquired by the data characteristics acquisition section; and a diagnosing section for conducting the diagnosis using both of the diagnostic data including appropriate parameter data acquired by the diagnostic data acquisition section, and the diagnosing techniques acquired by the diagnostic knowledge acquisition section; wherein the diagnosis device further includes an effective rate calculation section for calculating an effective rate of each diagnosing technique acquired by the diagnostic knowledge acquisition section; before conducting the diagnosis, the diagnosing section selects, of the diagnosing techniques acquired by the diagnostic knowledge acquisition section, an optimal diagnosing technique according to particular results of the calculation of the degrees of effectiveness of the acquired diagnosing techniques by the effective rate calculation section; the diagnostic knowledge further includes the number of application cases that denotes the number of application cases in which a combination of the data characteristics and one of the diagnosing techniques is effective; the diagnostic knowledge acquisition section acquires the number of application cases in addition to the diagnosing technique; and the effective rate calculation section calculates the effective rate from a product of two values, wherein one of the two values is a ratio between the number of elements in data characteristics input to the diagnostic knowledge storage device in order to acquire from the device a diagnosing technique fitting the data characteristics previously acquired by the data characteristics acquisition section, and the number of elements in the data characteristics contained in the diagnostic knowledge matching the input data characteristics, and the other value is the number of application cases that is contained in the diagnostic knowledge.