PURPOSE: To control exposure according to an exposure state and the position of a rotor.

CONSTITUTION: The rotational speed of the rotor 20b constituting an iris is detected by a rotation detection means such as a Hall element 20c. Rotor positional information Sr which is obtained from the rotation detection means and which shows the position of the rotor and a signal Sy which is generated according to light quantity passed through a diaphragm blade constituting the iris and which shows the exposure state are supplied to a diaphragm control means 27 consisting of a comparison means 27a, an addition arithmetic means 27b, a differentiation arithmetic means 27c and an integration means 27d. By controlling the exposure by executing digital arithmetic operation by the control means 27 and obtaining a driving signal Sb for driving the rotor 20b, the feedback quantity of the speed can be varied according to a diaphragm position. As the result, the resolution of the driving signal(the output value of a minimum bit) need not be set to be small.