To inexpensively, easily and highly precisely correct a sound speed value in performing location detection by using the time required for transmission of acoustic wave.
The time required since an optical signal for synchronization is received from a location instructing device 20 until an ultrasonic signal for distance measurement is received is measured for each reference body 10 (step S1), and the location coordinates of the location instructing device 20 are calculated by using a preliminarily set sound speed reference value V and the measured required time when the calibration of the sound speed value is not operated. When the calculated location coordinates are present in a preliminarily set calibration range, the sound speed value is presumed based on the distance until the preliminarily set calibration range and the required time used in calculating the location coordinates, and stored as a sound speed calibration value V' (steps S4 and S5). Afterwards, the location coordinates are calculated by using the sound speed calibration value V' in calculating the location coordinates (steps S1, S2 and S7).
Fujitsuna Hideyoshi
Osamu Suzawa