An aperture stop mechanism is composed of a small number of components, and accurately controls the position of a member that defines the rim of an aperture. The aperture stop mechanism (2) includes an aperture base plate (21), two aperture blades (22, 23), and a motor(24). The aperture blades (22, 23) are individually pivotable about pins (21a, 21b) provided on the aperture base plate (21). One aperture blade (22) has two gears (22a, 22b) formed along arcs about the pin (21a) with different pitch circle radii. The other aperture blade (23) has a gear along an arc about the pin (21b). The gear (22b) meshes with a motor gear (25), and the gear (22a) meshes with the gear (23a).