To provide a circuit breaker in which an overcurrent is interrupted by utilizing only the thermal deformation action of a bimetal without requiring the pressing force thereof, and in which an unstable state does not occur in the overcurrent interruption mechanism.

The circuit breaker includes an overcurrent interruption mechanism using a bimetal piece 11, an earth leakage interruption mechanism using a magnet coil, and a leakage test circuit 7 and a test switch 8 for testing the earth leakage interruption mechanism. Closing means 9 formed of the bimetal piece 11 is arranged in parallel with the test switch 8 on the leakage test circuit 7, and the leakage test circuit 7 is closed by thermal deformation of the bimetal piece 11 thus operating the earth leakage interruption mechanism.