body 11 provided on a lens barrel body fixed portion not shown the diaphragm blade 12 and a diaphragm driving ring 13 to be described later are contained within the diaphragm unit 11 another pin 12b of the diaphragm blade 12 is fitted in a cam groove 14 formed in the diaphragm driving ring 13 the shape of the cam groove 14 is determined so that the characteristic of the relation between the stroke of a stop down lever 17 to be described later and the variation in aperture step number becomes as shown by the standard set curve c of fig1 that is the shape of this cam is determined so that in an area x in the vicinity of the first step of stop down the stop down lever is displaced geometrically equal for the variation in aperture step number and in the succeeding area y the stop down lever is displaced arithmetically for the variation in aperture step number and that the stop down lever is smoothly displaced even at the boundary between the area in the vicinity of the first step and the succeeding area and this boundary point is set so that it lies in the vicinity of one aperture step the diaphragm driving ring 13 is rotatably fitted to the lens barrel body and is biased by a spring 15 so.