To provide an electromagnetic clutch forming an annular channel for reducing weight in a section in the outside in the radial direction more than a coil storage annular channel of a rotor and capable of preventing mud from being deposited in the annular channel for reducing weight even under the environment on which mud water splashes.

This electromagnetic clutch is provided with the rotor connected with the driving source through a belt and supported rotatably by a housing of rotary equipment, a coil stored in the coil storage annular channel formed in the rotor, and an armature fixed to an end part of a main shaft of the rotary equipment extended to the outside of the housing and attracted onto a bottom wall of the coil storage annular channel of the rotor by magnetic force generated by the coil to transmit power of the driving source to the main shaft. The annular channel for reducing weight is formed in a section in the outside in the radial direction more than the coil storage annular channel of the rotor 12a, and an outer peripheral wall 12f' of the annular channel for reducing weight is inclined in the direction of diameter expansion toward an open end of the annular channel from a bottom part of the annular channel for axial line of center of rotation of the rotor.