To enable the amount of movement of diaphragm blades to be increased and improve contrast.

The device includes: a diaphragm base 111 in which an aperture part 111a for passing light from a light source is formed; multiple diaphragm blades, such as diaphragm blades 112 and 113, which are provided movably in a direction perpendicular to an optical axis 101 of the light passing through an aperture 111a to form a diaphragm aperture 116 that is smaller than the aperture 111a; a diaphragm blade driving motor 114 for moving the multiple diaphragm blades to increase/decrease an opening amount of the diaphragm aperture 116; and a diaphragm cover 115 for forming a blade chamber 118 that houses the diaphragm blades 112 and 113 between the diaphragm cover 115 and diaphragm base 111. The blade chamber 118 is provided with a ventilation hole 119 for externally exhausting heat inside the blade chamber 118.