A spool 9 is connected to the rotating shaft 1 so that the spool 9 is slidable with respect to the shaft 1, while the rotating movement of the shaft 1 is transmitted to the spool 9. The spool 9 is formed with a first recess 9-3 of a reduced circumferential extension for obtaining a reduced compression capacity and a second recess 9-4 of an increased circumferential extension for obtaining a full capacity of the compressor. An intake pressure is always opened to a side of the spool, while an intermediate chamber is formed on the other side of the spool. A spring 10 is arranged for causing the spool 9 to be moved toward the intermediate pressure chamber. At the instant the compressor is brought into operation, introduction of the intake gas occurs via the first recess 9-4, thereby obtaining a compression capacity of 3 to 5% compared to the full capacity of the compressor.