To adjust the capacity of a compresser in plural stages by comprising a group of the first and second compressing cylinders respectively composed of two compressing cylinders, and opening and closing the first and second cylinder passages communicated with the group of the first and second compressing cylinders by the unload valves.

When the first and second unload valves (hereinafter valve) 70, 80 are not energized, the coolant flown into an intake chamber 51 from an external, is taken into a cylinder through a partially opened communication port 72a and a valve chamber 54, and only one cylinder among four cylinders, is used. Then when the communication port 72a is opened by energizing only the first valve 70, the discharge pressure acting in the second discharging chamber, is acted on a piston 83, and the communication port 82a is completely closed, thereby only two cylinders are used. Further when only the second valve 80 is energized, three cylinders are used, and when the both valves 70, 80 are energized, four cylinders are used.