To always fix a variant diaphragm in a correct direction by providing boss holes arranged asymmetrically on the variant diaphragm side and providing bosses which can be combined with the boss holes only when the variant diaphragm is set in the specified correct direction on a lens chamber side.

The variant diaphragm 6 has six boss holes 64 one-to-one corresponding to six bosses 76 provided at the variant diaphragm holding part 72 of the lens chamber 7. Six bosses 76 provided in the chamber 7 and six boss holes 64 provided on the diaphragm 6 are formed at mutually asymmetric positions with respect to an optical axis L, and can be combined with each other only in a state where the diaphragm 6 is set in the correct direction with respect to the chamber 7.