To provide a circuit breaker in which protection function of the breaker can be selectively switched over by a simple button operation so that it can be operated as a wiring breaker or an earth leakage breaker as required.

The circuit breaker includes an overcurrent protection and a ground fault protection function having, mounted thereon, a main circuit contact 2, a switch mechanism part 3, an operating handle 4, an overcurrent tripping device 5, a zero-phase current transformer 6, a leakage detection circuit 7, and a trip coil unit 8 in a body case. A mode switching unit 11 which makes On-Off operation by a push-button 16 is interposed and connected to a wiring path of a power supply line 9 that supplies power from the main circuit 1 to the leakage detection circuit 7 and selects and sets the ground fault protection function to "valid" and "invalid".