PURPOSE: To detect variations in the grounding resistance value easily and accurately by detecting unbalanced current with a balance bridge circuits each formed by a grounded circuit to be inspected and a grounded circuit for assisting inspection.

CONSTITUTION: A grounded circuit to be inspected and a grounded circuit for assisting inspection each with grounding resistances R1 and R2 forms a balance bridge circuit with a proportional arm due to an impedance of vector values Z1 and Z2 as formed thereby, an impedance variable in the vector value Z3, an impedance of a vector value Z4 and then, after the adjustment of the vector value Z3, no current flows through the impedance of the vector Z4. On the other hand, as the resistance R1 changes, the balance of the bridge circuit breaks and current flows through the impedance of the vector value Z4 thereby enabling easy and accurate detection of variations in the grounding resistance.