Positioning control relating to a two-aperture stop necessary for stereoscopic photography can be performed accurately and securely using a simple, inexpensive, compact, and lightweight structure. The apparatus comprises a two-aperture stop (28) provided with two stop apertures (28a, 28b), and a shutter plate (302) for opening and closing one of the two stop apertures. The two-aperture stop (28) is moved so as to intersect with an optical axis so that in monocular photography the two-aperture stop is positioned in a first position in which the stop aperture (28b) is disposed substantially at the center of the optical axis and so that in stereoscopic photography it is positioned in a second position in which the two stop apertures (28a, 28b) are disposed off-center from the optical axis. A drive mechanism comprising a cam plate (303), a spring (304), and a rotary solenoid (301) is used to move the shutter plate (302) so as to close the stop opening (28a) of the two stop apertures in the first position and to open both of the two stop apertures (28a, 28b) in the second position.
