PURPOSE: To improve control precision by calculating a control stop-down step value and detecting whether the calculated value is on an open-aperture side or small-aperture side, and varying a reference potential so that stop control is perfect or performing complete stop-down operation.

CONSTITUTION: The outputs of a film speed setting means 11 and a light metering means 10 are inputted to an arithmetic means 20, whose output is inputted to an arithmetic means 21 together with the output of a shutter speed setting means 14 to calculate the control stop-down step value. This calculated value is inputted to comparators 24 and 25 to detect whether the control stop-down step value is on the open-aperture side of an aperture value set near the open-aperture value or on the small-aperture value of the set stop-down step value. Gate means 30W32 are controlled according to the outputs of the comparators 24 and 25 to vary the reference voltage of aperture control.