A diaphragm electromagnetic drive device which comprises ring-shaped rotor and stator disposed coaxially about the optical axis of a taking lens and in which the rotor is rotated to thereby rotate diaphragm blades so as to vary successively a diaphragm diameter defined by the diaphragm blades. The mutually opposed surfaces of the rotor and stator are constructed respectively in the shape of a side surface of a truncated cone, so that the areas of the opposed surfaces can be increased without increasing the outside diameter and the length in the optical axis direction of the electromagnetic drive device.