PURPOSE: To improve the precision of automatic exposure control by reflecting totally the light, which is transmitted through a photographic lens and a main mirror, on a submirror and converging this light onto an aperture diameter detector unit and forming an image of diaphragm blades to detect the aperture diameter.

CONSTITUTION: The light from diaphragm blades of a lens 11 is transmitted through a main mirror 12 and is reflected totally on a submirror 13 and is led to an aperture diameter detecting unit 15 through an image-forming lens 14, and simultaneously, the image of diaphragm blades is formed onto the detecting unit 15. When the aperture of the photographic lens 11 is opened, the image of diaphragm blades is formed in the peripheral part of the detecting unit 15, and the output of the detecting unit 15 is changed rapidly in the peripheral part. According as diaphragm blades of the lens 11 are closed successively, the image-forming position of diaphragm blades is moved from the peripheral part of the detecting unit 15 toward the center successively and the position where the output of the detecting unit 15 is changed rapidly is moved from the peripheral part toward the center successively. Consequently, the aperture diameter is detected directly in accordance with the position where the output of the detecting unit 15 is changed rapidly.