PURPOSE: To facilitate maintenance and inspection of a system of a DC circuit by always measuring a ground insulation value of the DC circuit hot line to display measured values continuously.

CONSTITUTION: A digital meter relay 6 is connected between the positive pole and negative pole sides of a DC circuit and the ground through a manual operation switch 4 and a flicker relay 3 to measure leaked current. Every time a mobile contact of the flicker relay 3 is operated to switch within a specified time limit, the leaked current is measured as flowing through an insulation resistance 12 between the positive pole side and the ground or the insulation resistance 13 between the negative pose side and the ground. As arithmetic device 7 computes an insulation resistance value from the leaked current measured and a DC voltage nd the results are shown on a digital display device 8.