To provide a light quantity adjustment device that prevents the speed at which a diaphragm blade reaches a target diaphragm position from becoming too high and that eliminates the need for time required for vibration stops.

A light quantity adjustment device comprises: a rotary plate 103 pivotally supporting a light shield member 107; a motor 101 rotating and driving the rotary plate 103; a rotary plate 104 having a cam groove with which the shaft part of the light shield member 107 engages, and configured to rotate the light shield member 107 in the direction of opening/closing a photographing optical path by the cam action of the shaft part and cam groove when the rotary plate 103 is rotated; a motor 102 configured to rotate and drive the rotary plate 104; and a control circuit 10 that controls the drive of the motors 101 and 102. The control circuit 10 rotates the rotary plate 103 by driving the motor 101. When the light shield member 107 reaches a target light quantity position, the control circuit 10 drives the motor 102 without stopping the motor 101, and rotates the rotary plate 104 at the same speed as the rotary plate 103 in the same direction.