PURPOSE: To read directly the ground resistance of a DC circuit by providing a variable resistor between the positive electrode and negative electrode sides of an ungrounded type DC line and thus constituting a bridge circuit, and processing the obtained resistance value.

CONSTITUTION: A voltage E is applied between the positive and negative electrode side lines l1 and l2 of the DC circuit and ground resistances RP and RN on the positive and negative electrode sides are measured by a measuring device 3. The device 3 has a variable resistance VR connected between the line l1 and l2; its one terminal is grounded through a resistance R0 and the other terminal is connected to a meter M. Then switches SW1 and SW2 are turned on firstly and dividing resistances R1 and R2 are adjusted to set the meter M to 0. When the ground resistance RP is measured, the switch SW2 is turned on and the switch SW1 is turned off. Then, the relation among the resistances of the bridge circuit is as shown by an equation I , and a subtracter 11 and a divider 12 perform arithmetics to indicate the ground resistance RP on the meter M. The negative electrode side gound resistance RN is as shown by an equation II. In this case, FP shows the voltage drop across the resistance R0. Therefore, the ground resistance of the DC line is read directly on the meter and the circuit is simplified.