To prevent degrading of work accuracy and generation of backlash sound by providing a leaf spring with its base end fixed to a fore end of a plurality of guide part protruding from a magnetic polar surface and its free end to the opposite side of magnetic polar surface of armature, and providing a compression coil spring pressing the armature to the side of connected member meshed with the armature.

Under the non-excitation condition of electromagnetic coil 6, the tooth 9b of an armature 9 meshes with the tooth 11a of a connection plate 11 by the aid of spring force of a compression coil spring 13, a V-pulley 3 is connected to a housing 1 and retained stationarily through the armature 9 and a leaf spring 10; and a guide pin 7 and a field core 5. Under the excitation condition thereof, the meshing condition between the tooth 9b and the tooth 11a is released against the spring force of a compression coil spring 13, and the V-pulley can rotate freely. With such a construction as above, it is possible to dispense with a spline fitting part, and to prevent deterioration of working accuracy caused by backlash in the case of machining tool, and generation of backlash sound in the case of medical treatmetn machine.