To provide a light quantity adjusting device capable of forming an accurate exposure aperture by suppressing inclination of a blade member.

The light quantity adjusting device comprises: a substrate with an optical axis aperture; a plurality of blade members arranged around the optical axis aperture and overlapped like scales in a circumferential direction; a drive ring arranged to sandwich the blade members between the drive ring and the substrate and for opening and shutting the blade members; and drive means for rotating the drive ring. Each of the plurality of blade members comprises: a base end part having a pair of support shafts positioned outside the optical axis aperture and for engaging the substrate and the drive ring, respectively; a blade part for forming a diaphragm aperture by moving with respect to the optical axis aperture; and elastic means for pressing and energizing the base end part of each blade member to the substrate side or the drive ring side between the substrate and the drive ring.