To provide a circuit breaker capable of selecting a thermal electromagnetic relay or a full electromagnetic relay as a tripping device.

As tripping means for actuating an opening/closing mechanism part so that a movable contact point comes into a non-contact state relative to a fixed contact point, any one of a thermal electromagnetic relay and full electromagnetic relay can be built in, including a hole 152h which engages with a first rotary member that receives an external force based on the displacement of a movable part of the thermal electromagnetic relay to rotate around a first support point, and an end face part 152t which engages with a second rotary member that receives the external force based on the displacement of the movable part of a full electromagnetic relay to rotate around a second support point. A trip metal fitting 152 is provided as a third rotary member for rotating around a third support point.