a base plate which has a circular optical path aperture to construct a blade chamber between the base plate and a cover plate and is provided with four guide pins at positions where segments connecting the four guide pins form a square on a blade chamber side surface of the base plate;

an electromagnetic actuator which is provided with a rotor having a permanent magnet and is mounted to the base plate in such a way that the four guide pins provided between a rotary axis of the rotor and a center of the aperture are arranged, two on each side of a line connecting the rotary axis and the center as a symmetry axis, to make four output pins provided integrally with the rotor at preset angular intervals in radial directions to project into the blade chamber;

a first blade and a second blade, each having an aperture forming edge, a first slot, and a second slot, so that in the first blade, the first slot provided parallel to the symmetry axis is fitted to two guide pins located on one side of the symmetry axis, while in the second blade, the first slot provided parallel to the symmetry axis is fitted to two guide pins located on a remaining side of the symmetry axis, and so that the second slot of the first blade and the second slot of the second blade are fitted to a first output pin and a second output pin, respectively, of the four output pins and thereby a reciprocating operation is performed parallel to the symmetry axis in directions opposite to each other by a reciprocating rotation of the rotor; and

a third blade and a fourth blade, each having an aperture forming edge, a first slot, and a second slot, so that the first slot of each of the first blade and the second blade is fitted to one of the guide pins located on each side of the symmetry axis and so that the second slot of the third blade and the second slot of the fourth blade are fitted to a third output pin and a fourth output pin, respectively, of the four output pins and thereby the reciprocating operation is performed perpendicular to the symmetry axis in directions opposite to each other by the reciprocating rotation of the rotor,

the four blades being actuated by the rotation of the rotor so that a size of the optical path aperture is changed into a similar figure by cooperation of aperture forming edges of the four blades.