In order to reduce its size of a coil-rotation type electromagnetic clutch in an axial direction and to prolong the life of its brush, a sliding current supply mechanism, which has slip rings and slidable brushes which contact the slip rings, is located inside an inner circumference of a housing boss of a compressor. The current is supplied to an electromagnetic coil via the sliding current supply mechanism. Since the sliding current supply mechanism is located in the boss, a current supply passage is formed without increasing the size of the clutch in axial direction. Thus the installation of the electromagnetic clutch in the engine room is facilitated. Furthermore, since the sliding current supply mechanism is located near the rotational center of the clutch, the life of the brush is prolonged due to the small peripheral velocity of the brush.