PURPOSE: To prevent the generation of an arc between a breaker main body and an installation stand when the breaker main body is removed from the stand, by providing a specific means to a safety trip device, in a plug-in circuit breaker which consists of a breaker main body, an installation stand, and a safety trip device.

CONSTITUTION: This breaker is composed of a breaker main body 1 having a switching contact shoe, a trip mechanism, and a contact shoe member for connecting; an installation stand 2 having terminal members connecting with external wires and contactable with the contact shoe member for connecting, and installing the breaker main body 1; and a safety trip device. In the safe trip device, lever means 13 and 15 having the first abutting means 13a to abut to the breaker main body 1 at the installation stand 2 side, and the second abutting means 15b to abut to a trip operation member, are provided to the trip operation member to operate the trip mechanism when it is moved to the breaker main body 1 side, in the direction of the stand 2, and displaced in the direction of the stand 2, while a lever ratio to amplify the displacement amount of the first abutting means 3a, and to displace the second abutting means 15b, is given to the lever means 13 and 15.