PURPOSE: To hardly form a ghost on a photographic image plane by slanting and fitting a stop unit which has a diaper diaphragm opening so that one side of the diaper diaphragm opening is almost parallel to the photographic image plane.

CONSTITUTION: The diaphragm unit D detects the brightness of external light by a photosensor and the output shaft 12 of a galvanomirror M rotates corresponding to its output. A swing lever 11 which is coupled integrally with the shaft, therefore,rotates and then two diaphragm blades 9 and 9' move in opposite directions in parallel to perform opening/closing closing control over the diaphragm diameter A. Namely, the diaphragm unit D moves in a direction X-X to vary the diaphragm diameter A, but this direction X-X slants at θ to a horizontal axis Z-Z or a guide member and one side of the diaper diaphragm opening A is nearly parallel to the horizontal axis Z-Z. Consequently, and image which has no ghost is obtained.