A circuit breaker includes a main case; a switching mechanism accommodated in the main case, and having a mechanical part for opening and closing a circuit; a trip mechanism configured to trigger the switching mechanism to a circuit interrupting position upon detection of an abnormal current on a circuit; and an external device detachably mounted to the main case, wherein the circuit breaker includes an interrupting means configured to interrupt the switching mechanism by contacting at least a part of the external device when the external device is mounted to the main case.