PURPOSE: To prevent the deformation of a stopper piece when interior accessories, having a great shock from a ratch, are attached, in an opening and closing mechanism in which the stopper piece, to which the ratch is abutted at the time of trip operation, is foldedly formed integrally with a side plate.

CONSTITUTION: A stopper pin 24 is mounted on a side plate 5, and a ratch 7 is made to be doubly abutted to a stopper piece 5b and the stopper pin 24 formed on the side plate 5, only when interior accessories like an undervoltage tripping device, etc., are attached. This divides a shock load from the ratch 7 into two to eliminate the deformation of the stopper piece 5b. The stopper pin 24 can be detachably inserted in a hole of the side plate 5 to be made simply addable as required.