PURPOSE: To simplify the structure, by constituting a store-energizing member for store-energizing in the direction for energizing a member which executes a reciprocating motion between an exposure position and a nonexposure position, and a hooking member for detaining its advancing motion to the exposure position, of the same member.

CONSTITUTION: A diaphragm control level 80 and a diaphragm interlocking lever 79 are reciprocating motion member which take a going-up position in case of a nonexposure operation and take a going-down position in case of an exposure operation, are energized in the going-down direction by a spring 82, and the going-down is stopped by a diaphragm holding lever 75 and a holding release lever 102. When stopping down, a cam 71 is turned to the right, detaining of the lever 75 is released, and when opening the diaphragm, the cam 71 is turned to the left, the lever 79 goes up, energizing force is store-energized to the spring 82,also the levers 75, 102 are turned to the right, and going-down of the levers 80, 79 is stopped. That is, the cam 71 serves both as an action for releasing the detaining of the levers 75, 102 and an action for store-energizing the energizing force to the spring 82, therefore, the whole structure of the diaphragm mechanism is simplified.