PURPOSE: To prevent generation of chattering noise and enable sure lubrication by providing a switch valve opening a pressure introducing passage when the difference of pressure of an operating room and a high pressure chamber becomes over a set pressure and providing a lubricant passage introducing lubricant of the high pressure chamber to a back pressure passage.

CONSTITUTION: A switch valve 22 opening a pressure introducing passage 21 when the difference of pressure of an operating room 17 and a high pressure chamber 7 becomes over a set value is provided. And a lubricating passage introducing lubricant of the high pressure chamber 7 and made up by a communicating groove 14 formed on an end face of a rotor 8, an axial hole 15 and a through hole 16 provided on a rear plate 5 is provided on a back pressure passage 13. Then, when starting, coolant gas of the operating room 17 can be introduced to a back pressure passage 21 through the pressure introducing passage 21, so back pressure is surely applied to a vane 12 and generation of chattering noise due to striking of the vane immediately after starting can be prevented. In normal operation, high pressure lubricant is introduced to the back pressure passage 13 from the high pressure chamber through the lubricant passage, and lubrication of the vane 12 and respective parts can surely be made and the whole durability can be improved.