PURPOSE: To simplify a mechanical and electrical mechanism for controlling exposure by using a single-speed shutter used also as a diaphragm.

CONSTITUTION: The diaphragm consisting of at least two shutter blades 11 and 12, a diaphragm opening and closing means 15 which is pivotally supported in freely turnably in order to open or close the diaphragm and always elastically pressed in a closing direction at the same time, a shutter driving means 17 which drives a shutter by releasing operation, an oscillating pin 19 which is projectingly provided on one end of an oscillating lever whose other end is pivotally supported by the driving means 17 and which moves the opening and closing means 15 in an opening direction by resisting to the elastic pressing force by moving in an identical direction according to the movement of the opening direction of the driving means 17 and returns it in the closing direction by detaching from the opening and closing means 15 in the midst of the movement of the opening direction of the driving means 17 and a detaching position varying means 111 which changes the detaching position of the opening and closing means 15 by the pin 19 according to the value of an exposure decision factor are provided.