A diaphragm blade is rotatably secured by a rotational shaft disposed in a base. The aperture blade has a diaphragm aperture at the central portion thereof. A shape-memory alloy which deforms in accordance with temperatures and an elastic member are respectively secured to the end portions of the aperture blade. The other end portion of each of the shape-memory alloy and the elastic member is secured to the base. The shape of the shape-memory alloy is deformed by changing the temperature so that the aperture blade is displaced. In consequence, a diaphragm aperture formed in the aperture blade is positioned/retracted from the optical path so as to change the quantity of light which is transmitted through the optical path.