PURPOSE: To prevent abrasion of vane by providing a pressure control valve for holding the differential pressure between intake pressure and the lubricant pressure in back pressure path at predetermined level in a lubrication path communicating between intake section and back pressure path thereby maintaining the lubricant pressure in back pressure path at proper level.

CONSTITUTION: Under relatively low rotation of rotor 12 where the differential pressure between intake pressure and delivery pressure is low, the differential pressure between the back pressure path 15 and a pump chamber 17a located in the intake stroke or the differential pressure between the chambers 22, 26 is also low thereby the ball valve 33 in pressure control valve 32 is resiliently pressed against the seat face through a spring 35 thus to close the lubrication path 31. The lubricant sucked from the high pressure section 7 into the back pressure path 15 will return to the pump chamber 17a only through a sliding gap 28. Consequently, the working oil in the back pressure path 15 is held at proper level resulting in prevention of abnormal abrasion of vane 17 or cam ring 3 or noise to be produced through landing of vane 17.