PURPOSE: To improve dividing accuracy in the opening/closing space of aperture blades with a simple constitution by making the reduction ratio between a gear integrally provided with a turning part and a screw shaft provided on the output shaft of a driving part.

CONSTITUTION: Interlocking lever pins 3a and 3b are provided projectingly on both ends of an linking lever 3 and relatively slidably inserted into holes long sideways 1b and 2b pierced in the end parts of the aperture blades 1 and 2, so that they are connected to the linking lever 3. Moreover, a guiding rail for guiding the movements in the directions of the arrows of the aperture blades 1 and 2 is formed in a base plate and an aperture for an optical path which is opened or shielded with the respective apertures 1a and 2a of the aperture blades 1 and 2 is pierced in the base plate. An output worm gear 10 is engaged with the gear part 3f of the linking lever 3, fixed to be coaxial with the magnet rotor part of a motor driving part 13 and rotatably supported by a bearing provided in a holder 12 in a shape like an U-shaped frame.