To accurately detect a buried object by preventing an erroneous detection due to noise.

A reception signal when no signals are transmitted and that when signals are transmitted are recorded by a recording device 22, and a differential signal A where a representation signal B of the reception signal when no signals are transmitted is subtracted from a representation signal (A+B) of the reception signal when signals are transmitted is calculated by a computer 23 for displaying an underground state at a display part 24. More specifically, the representation signal of the reception signal for a plurality of times being recorded by a recording means when no signals are transmitted from a transmission antenna is compared with the representation signal of the reception signal for a plurality of times being recorded by the recording means when signals are transmitted from the transmission antenna, and the difference between both the signals is displayed, thus eliminating a reception voltage being generated by external noise or noise depending on the inside of equipment, and hence preventing erroneous detection due to the noise for accurately detecting a buried object.