To provide a diaphragm adjusting device capable of having a diaphragm switching mechanism for the size of the outside diameter of a photographic lens.

A groove 28b and a cut 28c are formed in a lens-barrel part 28a of a lens holder 28. The diaphragm openings of a stationary diaphragm plate 30 and a movable diaphragm plate 31 are formed to a star shape. A tongue piece 30a formed at the stationary diaphragm plate 30 is detained into the groove 28b to position and fix the diaphragm opening. The movable diaphragm plate 31 is freely rotatably assembled to the inner side of an arcuate projection formed on the back of a first lens. A connecting lever 31b is projected from the cut 28c and is connected via a connecting plate 34 to a rotating member 24b of an ammeter 24. A magnetic field is generated in a coil 24c according to the detection value of an external light quantity to generate magnetic poles in magnetic pole parts 24f and 24g. A permanent magnet 24a and the rotating member 24b rotate to rotate the connected movable diaphragm plate 31 around the photographic optical axis. As a result, the opening are of the diaphragm formed by the diaphragm plates is changed.