The space required in the vicinity of a shutter aperture can be reduced to achieve an overall reduction in size of a device by providing a diaphragm mechanism for covering the shutter aperture formed of a plurality of members cooperating with each other to cover the shutter aperture. By cooperation of a small-diaphragm plate having a diaphragm aperture formed therethrough smaller in diameter than the shutter aperture and an auxiliary plate, for example, a space reduction is facilitated. The small-diaphragm plate is pivotably mounted adjacent one side of the shutter aperture. The auxiliary plate has a shape that cooperates with the convex-arcuate portion to cover the shutter aperture. When the small-diaphragm plate and the auxiliary plate are driven by the diaphragm actuating member and moved to a first position covering the shutter aperture, the two plate cooperate to fully cover the shutter aperture. In a second position retracted from the shutter aperture, the space around the shutter aperture is reduced because the small-diaphragm plate is small in size.