To provide a light quantity adjusting device operating a shutter blade and an ND filter by a common electromagnetic drive source and, when the ND filter is opened/closed, preventing a stop of an incident light quantity from an opening by the opening/closing operation of the shutter blade.

An ND filter 4 is retained by a cam groove 3g with the ND filter 4 entering an opening part 3a; by energizing an actuator 1 at a third position in the positive direction, a cam shaft 7a moves a cam surface to a fourth position and the ND filter 4 rotationally turns; and, by de-energizing the actuator 1 at the fourth position, the cam shaft 7a moves the cam surface to a first position by urging force of a spring 8 and the ND filter 4 rotationally turns in the direction of retracting from the opening part 3a. The shutter blades 5 and 6 rotationally turn in the direction of closing the opening part 3a by inversely energizing the actuator 1.