PURPOSE: To most suitably control the capacity according to an amount of air used by a method wherein a suction throttle system and an ON-OFF system are compared with each other and selected by using a microcomputer.

CONSTITUTION: If the suction throttle system is selected by the microcomputer 35, an electromagnetic valve 29 is opened by means of the microcomputer 35. If a decrease of the amount of the air used causes a pressure of a cooler 23 to be increased, a secondary pressure of a pressure adjusting valve 30 acts on a throttle valve 25 to throttle the amount of the suction air of a compressor. If the ON-OFF system is selected, the microcomputer 35 closes the electromagnetic valve 29. When a pressure of the cooler 23 reaches the upper limit, a pressure switch 34 turns a three- way electromagnetic valve 27 off, the pressure of the cooler 23 effects opening the throttle valve 25 to its full width through a pressure reducing valve 28 and a circuit 26b, and at the same time an air release valve 33 is opened to discharge a pressure of a separator 22 and to permit an unload operation to be performed. When the pressure of the cooler 23 reaches the lower limit, the 3-way electromagnetic valve 27 is turned ON, the pressure of the cooler 23 effects opening the throttle valve 25 to its full width through the pressure reducing valve 28 and a circuit 26a and at the same time, the air release valve is closed to make a full-load operation be performed.