An optical performance is deteriorated by a minute phase difference of transmission wave front of an ND filter. In a light quantity adjusting device which includes an ND filter (P1) for attenuating a quantity of light passing through an aperture formed by diaphragm blades (S11 and S12) and in which a cover ratio of the aperture by the ND filter (P1) is changed, setting is conducted such that a phase difference between lights with a predetermined wavelength λ which pass through regions (N11, N12, and N13) of the ND filter (P1) with different transmittances inside the aperture becomes substantially 0λ, substantially 1λ, or substantially 2λ in a state in which the aperture is completely covered with the ND filter (P1).