Diaphragm blades move in a direction approaching to or separated from an optical axis of an incident light at the time of operation. Opening edges of the diaphragm blades are provided with neutral density (ND) filters having a predetermined transmittance to pass the incident light. Edges of the ND filters are provided with inclined surfaces having a predetermined angle with respect to the optical axis. With this configuration, flare is suppressed and image quality is enhanced.