PURPOSE: To aim at preventing inferior compression upon starting in a low rotational speed range, by providing such an arrangement that a valve member is abutted against a valve seat under the urging force of a spring when a difference in pressure of a compressor is small so that a suction port is maintained to be blocked to surely press a vane against the inner wall of a cylinder.

CONSTITUTION: When a compressor is started in such a condition that a difference in pressure of the compressor is zero or low since the compressor is rested for a long time, the pressure in a space 30 becomes equal to the pressure in a suction passage 19. Accordingly, a valve element 23 is abutted against a valve seat 22 under the resilient force of a spring 29 to hold the blocking of the suction passage 19. Therefore, the pressure in a suction side working chamber 8 lowers due to increase in the volume of the working chamber 8 accompanied with rotation of a vane 3 so that a force acting upon the tip end of the vane 3 to introduce the vane 3 into a vane slot 4 become remarkably small. As a result, the vane 3 is pressed against the inner wall of a cylinder 1 so that bad affection of the vane may be prevented to sustain normal operation.