PURPOSE: To prevent early wear of an engagement clutch means by combining a friction transmitting mechanism with the engagement clutch means while downsizing and weight-reducing an electromagnetic clutch.

CONSTITUTION: A compressor is constituted to be of variable capacity type. A pin 44 constituting an engagement clutch means 43 is supported by an armature which is integratedly rotated with a rotary shaft 17 of this compressor. A pin hole 50 is formed in a rotor 34 opposite thereto. When the compressor 2 is started, the capacity of the compressor (the driving torque) is reduced, and at the same time, the rotation of the compressor is started by making the magnetic attracting force of a magnet coil 32 smaller than the set value, thereby attracting the armature only, i.e., with the frictional transmission only. Around the time when the rotation of the compressor has been increased sufficiently, the pin 44 is also attracted and engaged with the pin hole 50 by energizing and controlling so that the magnetic attracting force of the magnetic coil 32 may be larger than the set value.