PURPOSE: To obtain precise diaphragm diameter information and to facilitate the design of a device by guiding the luminous flux of a diaphragm diameter optical detection system side which is obtained by splitting light passed through a photographic lens by a beam splitter onto a diaphragm diameter element, and forming the image of a blade.

CONSTITUTION: Light from the diaphragm spring of a photographic lens 1 passes through the beam splitter and is reflected totally by a submirror 3 and guided through an image forming lens 4 onto the diaphragm diameter detecting element 5 to form the image of the diaphragm blade. The position where the output of the detecting element 5 varies abruptly is detected to detect an aperture diameter directly. Thus, the diaphragm diameter is detected with high precision. Further, a driving system for the blade and a measurement system are separated from each other, so the measurement system is arranged relatively freely to increase the degree of freedom of the design.