A diaphragm device including a pair of diaphragm blades movable in opposite directions relatively to each other along a moving line passing through an optical axis of an optical system characterized in that each of the diaphragm blades is provided with an opening of a shape consisting of combination of an arc having a center of curvature located on said moving line and at least one arc having a radius smaller than the above-mentioned arc and having a center of curvature located on said moving line or on a line at right angle to said moving line.