To prevent the malfunctioning phenomenon or compression defect phenomenon of a vane at the time of starting a compressor by applying the constitution that a piston is provided at the end position of compression stroke, so as to be axially slidable, depending on a pressure difference between a high-pressure fluid and a low-pressure fluid, and groove length is made variable, regarding a groove to feed lubricating oil to vane back pressure space.

When a compressor is started in the condition where no pressure difference exists between a high-pressure fluid and a low-pressure fluid, a force transmitted from a piston 14 to a compression chamber is small. As a result, the piston 14 comes to be positioned at the side of the compression chamber in a sliding chamber 18, due to the resultant force of a force introduced to a piston back pressure chamber 17 via a back pressure feed passage 16, and the force of a spring 15, thereby closing a groove 12 at the end position of compression stroke. Thus, the length of the groove 12 at the end of the compression stroke becomes short and space not communicated to vane back pressure space 13 becomes long. Consequently, when a pressure difference comes to exist between the high-pressure fluid and the low-pressure fluid, due to the start of the compressor, high-pressure lubricating oil flows into an oil feed passage 11 and the internal pressure of the groove 12 increases. A vane 4 is threrby forced out and a normal compression operation starts.