To detect the initial position of an aperture blade with a simple constitution, as to a diaphragm device.

The diaphragm device is provided with the aperture blade 20 for stopping down the exposure aperture part 10a to a desired diameter, and a stepping motor 50 for driving the aperture blade, and in the case of detecting the initial position of the aperture blade 20, a detection value as to information on the light quantity detected by a light receiving element 63 for detecting the quantity of light passing through the aperture part 10a whenever necessary is compared with a prescribed storage value in a storage part 65 where the information on the quantity of the light passing through the aperture part in accordance with the operating position of the aperture blade 20 is previously stored whenever necessary, and it is judged that the aperture blade 20 lies at the initial position when both values become equal.