A light amount adjuster for adjusting a size of a diaphragm aperture on an optical axis of an optical system, includes: a pair of straight moving diaphragm blades so provided at a portion to which the diaphragm aperture is opposed as to slide along a plane orthogonal to the optical axis in a direction of getting close to or separated from the optical axis; a pair of swing diaphragm blades so provided at a portion to which the diaphragm aperture is opposed as to swing about a center axis in parallel to the optical axis in a direction of getting close to or separated from the optical axis; and a drive mechanism for adjusting the size of the diaphragm aperture by sliding the pair of straight moving diaphragm blades and by allowing the pair of swing diaphragm blades to swing.