To explore foundations in a short time without having to directly determine the detection time difference of Rayleigh waves between detectors.

When a power amplifier 50 actuates a shaker, a foundation is excited, and Rayleigh waves are detected by the detectors. Sine multiplication signals Ms acquired by multiplying their detection signals by sine wave signals Ss at a multiplier 32S and cosine multiplication signals Mc acquired by multiplying the detection signals by cosine wave signals Sc at a multiplier 32C are each compared in magnitude with saw tooth waves of, for example, 256 times the frequency of the sine wave signals Ss at comparators 33S and 33C. Outputted binary signals Bs and Bc are passed through gate circuits 34S and 34C only for a period of prescribed wave numbers of the sine wave signals Ss to control counters 35S and 35C. The counters 35S and 35C count pulses of, for example, 2,048 times the frequency of the sine wave signals Ss. A CPU computes phase delays of detection signals to the sine wave signals Ss on the basis of their count values Cs and Cc and computes the detection time difference of Rayleigh waves between the detectors.