To adjust light quantity by effectively avoiding the deterioration of MTF caused by small-aperture diffraction with a relatively simple means.

A light quantity adjusting device holds ND filters 62 and 72 which include light reducing parts 62b and 72b respectively, and light shielding parts 62c and 72c respectively. In a pair of holding frames 60 and 70, the directions in which the transmittances of each of the light reducing parts change are opposite to each other, and the arrangement directions of the light reducing part and light shielding part in each of ND filters are also opposite to each other. The pair of holding frames is moved in such a manner that the light reducing parts and the light shielding part do not overlap in an aperture until the aperture diameter of an aperture stop changes from a maximum diameter to a minimum diameter, and that the light reducing and shielding parts cover the aperture subsequently when the aperture diameter of the aperture stop reaches the minimum.