Insertion type circuit breaker

A Plug-In Circuit Breaker comprises: a trip rod (21) that brings in operation a switch contact making and breaking a circuit where a load current flows, and a trip mechanism that causes this switch contact to be in an open circuit; a lever (22) that acts on the trip rod (21) when plugging a circuit breaker body (1) in a mounting base (7) or detaching the circuit breaker body (1) from the mounting base (7); an elastic member (23) giving an impetus to this lever (22); and a lock mechanism (30) that locks the lever (22) against the bias force given by the elastic member (23) when the circuit breaker body (1) is detached from the mounting base (7). As a result, it is possible to carry out an open/close check easily even in the state that the circuit breaker body (1) is detached from the mounting base (7). In addition, it is possible to obtain a Plug-In Circuit Breaker performing a tripping operation even in the case of plugging the circuit breaker body (1) in the mounting base (7), or in the case of detaching the circuit breaker body (1) from the mounting base (7) in the state that the trip rod (21) is made stationary.