PURPOSE: To facilitate setting, by shifting rotation positions freely by a required quantity to set two transmitting materials.

CONSTITUTION: When the upper end of an aperture value adjusting lever 11 is turned in the direction of an arrow, the turning force is transmitted to an aperture value converting member 1 to change the aperture value. A proper turn resistance is added to the turning part of this aperture value adjusting lever 11, and this lever is held in a desired aperture value position. A spring 10 draws always the aperture value adjusting lever in one direction to make the setting of the aperture value safe and facilitates the restoration to the home position. The aperture value adjusting lever 11 and the aperture value converting member 1 are so adjusted that their turning positions are related to each other accurately.