To probe the ground more precisely by making influence due to noise to be less likely.

Current is supplied between a pair of current electrodes 3, installed at a ground 2 to be measured at a fixed period for measuring the potential difference between the potential electrodes 4 installed at the ground 2 to be measured, the difference in the amplitude and phase of the waveform of the potential difference to the waveform of the current is obtained, and the relations between the frequency and amplitude and that between the frequency and phase difference are obtained, by performing measurement for a plurality of times by changing the frequency of the period of current, thus discriminating the ground to be measured based on the relations.