To obtain a ground-fault circuit breaker that shortens the time from the detection of a leakage to the breaking of feeding lines.

An opening and closing part 1, which opens or closes feeding lines L1, L2, a leak-detecting IC2 which detects leakage based on the unbalanced current of a zero-phase current transformer ZCT and outputs a leak detection signal Vx, and a control device 3, which in responsence to the leakage detection signal Vx, turns off normally-closed relay contacts r1, r2 inserted in the feed lines L1, L2 are stored inside a device unit 10, in which a pair of cables 9-1, 9-2 is attached. Once the output of the ZCT exceeds a prescribed level, the IC2 outputs a Vx immediately. As a result, the control device 3 controls the opening and closing part 1 and opens the feeding lines L1, L2, so that in comparison to prior art, the time from the detection of a leakage to the breaking of the feeding lines can be shortened.