PURPOSE: To increase the effective area of a magnet and a coil, to simplify the structure and also to improve the assemblage of it by having such a constitution that an opposite surface between a rotor and a stator is formed in the side shape of a circular truncated cone.

CONSTITUTION: An electromagnetic driving device for changing the diaphragm diameter of an iris diaphragm constituted of three stop blades 10 is mainly constituted of the rotor 20 and the stator 30. The magnet 24 and the coil 34 are respectively disposed on the opposite surface between the rotor 20 and the stator 30 so that the magnet 24 and the coil 34 may incline with reference to an optical axis L. As to the attachment of the magnet 24 of the rotor 20, plural divided magnets 24 are respectively incorporated in a magnet positioning hole parts on the rotor 20. The device is constituted so that the opposite surface between the rotor 20 and the stator may be formed in the side shape of the circular truncated cone, so that the effective area of the magnet 24 and the coil 34 can be increased.