PURPOSE: To lower the power consumption of a compressor, by changing the volume of a cylinder for performing a net compression work of the compressor to meet with required capacity of the compressor varied at every moment.

CONSTITUTION: When a servomotor 16 is turned counterclockwise by a signal from a control circuit, a spur gear 18 is turned clockwise, and spur gears 21R, 21F mounted on the same turning shaft 19 as the spur gear 18 are also turned in a clockwise direction. This causes variable rings 22R, 22F to turn clockwise by the aid of teeth 24R, 24F formed around their peripheral portions, so that the capacity of a cylinder for performing compression work is increased. On the other hand, when the servomotor 16 is turned clockwise, the volume of the cylinder for performing compression work is decreased. By thus changing the cylinder volume according to the required capacity of a compressor, it is enabled to lower the power consumption of the same.