To provide a fall-off prevention structure of a plug-in breaker capable of firmly fitting the plug-in breaker to a fitting plate.

A fitting plate 1 having a hooking part 15, a plurality of conductive bars 2 to 4, and rest blades 12, 13, to plug in for connection to the conductive bars 2 to 4 are arranged on a front face, and at the same time, a plug-in breaker 5 having a hooked part 16 formed for preventing movement of in a height direction of the fitting plate 1 by being hooked to the hooking part 15 with the rest blades 12, 13 in a state plugged into the conductive bars 2 to 4 for connection is provided. A pendent piece 25 is fitted to the fitting plate 1 pending toward a side contrary to a loading side, to which a turning operation part 30 and a fall-off prevention member 27 is fitted having a stopper part 26 freely moving back and forth between a position protruding against the loading side surface of the fitting plate 1 and a position not protruding, the stopper part 26 locked to the plug-in breaker 5 by having the fall-off prevention member 27 turn.