PURPOSE: To detect fault current either on the higher potential side or on the lower potential side of the dividing point, by providing a rectifier, from which fault current flows out or into which fault current flows in with respect to the dividing point of direct-current power source divided to a certain value, between the dividing point and the grounding member by way of an operational amplifier.

CONSTITUTION: From terminal Tc of a battery B, each one end of each parallel circuit of R1 and R2, diode D1 and R3, and diode D2 and R4 is connected, while the other ends are grounded. When the positive side circuit of the battery B leaks current, the current flows from the ground to R3, diodes, R1 and terminal Tc of the battery B, and a voltage drop of Ebc is generated between both terminals (b) and (c) of R3. When the negative side circuit of the battery B leaks current, the current flow from terminal Tc to R1, D1, R2, and to the ground, and a voltage drop of Eac is generated between both terminals (a) and (c) of R2. Such voltage drop is supplied into an amplifier AP. After amplification, when exceeding a certain input value, a comparator LM operates, finally actuating a relay Ry. In this constitution, meanwhile, an operational amplifier CAP is made up of the amplifier AP and comparator LM.