Abstract: Two acceleration detectors (11A and 11B) are dispose at a distance L from each other on the ground. A vibration exciter (15) vibrates the surface of the ground in the vertical direction to generate surface waves. A measuring instrument (12) includes a seismometer unit (12-1) for receiving detection signals from the two acceleration detectors to generate acceleration time-series signals, which are analog, and an A/D converter unit (12-2) for converting the analog acceleration time-series signals into digital acceleration time-series signals. The measuring instrument is connected to a personal computer (13) for receiving the time-series signals and then processing the signals on the basis of a predetermined analysis program, thereby calculating a mean propagation velocity Vr(f) and a depth D(f).