A light quantity adjusting device includes a substrate having an optical path aperture, a blade located on the substrate to adjust the quantity of light passing through the optical path aperture, and an electromagnetic driver that drivingly opens and closes the blade. The electromagnetic driver includes a rotor shaft, an exciting coil, and a frame that rotatably bears the rotor shaft. The rotor shaft includes a pointed first shaft end and a cylindrical second shaft end. The frame has a conical bearing recess that supports the first shaft end and a cylindrical bearing hole that supports the second shaft end. The electromagnetic driver is located so that the first shaft end of the rotor shaft is closer to the substrate than the second shaft end. An arm is coupled to the first shaft end of the rotor shaft to transmit an opening and closing driving force to the blade.