PURPOSE: To eliminate the difference of a position servo between its forward and backward directions due to hysteresis by displacing a diaphragm unit in the opposite direction by a correction quantity for canceling hysteresis only in one driving direction of the diaphragm unit.

CONSTITUTION: An aperture quantity command is set by a command potentiome ter 11 and an actual aperture value is set by a detection potentiometer 12. Outputs of those command potentiometer 11 and detection potentiometer 12 are inputted to a servoamplifier 15 to drive a motor 14 with its deviation output. A correcting circuit block 16 identifies the rotating direction of the motor 13 and causes displacement in the opp;osite direction by a correction quantity for canceling mechanical hysteresis only in one rotating direction of the motor 13.