Abstract: An electromagnetic clutch has an electromagnetic coil 3, an armature 5, and a rotor 1 of a substantially C-cross section, which is constructed by an outer ring 21 arranged outside of the coil 3, an inner ring 22 arranged inside of the coil 3, and a friction face 23 for connecting the inner and the outer rings 21 and 22. The outer ring has an inner periphery of a reduced diameter located nearer to the friction face, so that a thick wall portion 27 is created at a location adjacent the friction face 23. As a result, the magnetic resistance of the outer ring 21 is reduced at a location adjacent the friction wall 23, thereby increasing the total magnetic force of the magnetic circuit, thereby increasing an engaging face between the rotor and the armature.