PURPOSE: To increase the number of the steps of a diaphragm without enlarging a setting space by controlling incident light quantity by means of diaphragm plates and an ND filter.

CONSTITUTION: The ND filter 10 which reduces the incident light quantity in response to its density, the diaphragm plates 12 provided with diaphragm holes which reduce the incident light quantity in a proportion different from the ND filter 10 and electromagnetic actuators 14 and 16 which selectively or simultaneously insert the ND filter and the diaphragm plates 12 into pre scribed positions are provided. Therefore, the ND filter 10 and the diaphragm plates 12 can simultaneously be inserted into the prescribed positions by the electromagnetic actuators 14 and 16, and in this case, a target diaphragm value can be controlled after the diaphragm values of the filter and diaphragm plates are multiplied without increasing the number of the diaphragm plates 12. There fore, without increasing the number of the diaphragm plates, that is, without enlarging the setting space, the number of the steps of the diaphragm can be increased.