To obtain a light quantity adjusting device capable of decreasing gear noise caused by an influence of backlash even if a stepping motor is used as a drive source.

The light quantity adjusting device includes: three or more diaphragm blades; a ring member having a blade drive pin, and driving to open and close the diaphragm blade by rotating centering an optical axis; a drive source coupled with the ring member through a gear to drive the ring member; and an energizing gear meshed with the ring member through the gear, and energizing the backlash of the ring member. The energizing gear comprises a gear, and is energized in a rotating direction and an optical axis direction by an energizing member. The energizing gear is meshed with the gear of the ring member, thereby energizing the ring member in two directions, that is, the optical axis direction and the rotating direction.