An elastic wave is applied to an object under the ground, so that an elastic wave which varies in accordance with the material and shape of the object under the ground is detected and the detected elastic wave is collated with characteristic data such as elastic wave data, or the like, of various objects registered in advance to thereby identify/discriminate the object under the ground. A vibration generating probe and a vibration receiving probe, or, alternatively, a vibration generating and receiving probe is put into the ground so as to come in contact with an object under the ground. Then, the vibration generating probe or vibration generating and receiving probe is hammered to generate an elastic wave. At least one of one longitudinal wave and two transverse waves of the elastic wave which has propagated to the object under the ground is received by the vibration receiving probe or vibration generating and receiving probe. The vibrations of the elastic wave are converted into an electrical signal by a vibration sensor, and frequency response characteristic (and/or time-response characteristic) of the elastic wave is detected by a signal processing portion or by a signal processing portion and a data processing portion. The characteristic data is compared/collated with elastic wave data of various objects under the ground collected in advance, so that an unknown object under the ground is identified/discriminated.