Disclosed herein is a light quantity adjusting device including: first and second diaphragm blades shiftably provided to adjust a size of a diaphragm aperture; a neutral density filter blade provided shiftably in the same direction as the first and second diaphragm blades and having a neutral density filter adapted to cover the diaphragm aperture to reduce a quantity of light passing through the diaphragm aperture; and a drive mechanism adapted to shift the first and second blades and the neutral density filter blade; wherein the drive mechanism is configured to include an actuator, a first cam groove, a second cam groove, a third cam groove, an arm, a first cam-pin, and a second cam-pin.