The magnetic attraction force of an electromagnetic coil in an electromagnetic clutch is increased in a limited installation space, thereby improving reliability of operation. The dimensions of a field core housing the electromagnetic coil and a rotor are optimized to increase the magnetic attraction force. The thicknesses of cylindrical portions on the inner-diameter side of the field core and the rotor have larger thicknesses than those of cylindrical portions on its outer-diameter side so that the sectional areas of both cylindrical portions will be equal to each other. Also, a rotation transmission device provided with an electromagnetic clutch is proposed which can avoid deterioration of its sealing property and damage to its bearing and avoid cost increase.