PURPOSE: To assure a good lubricating characteristic even in such an operating range as a small volume of oil is discharged and to make an operation of the pump possible for a long period of time by a method wherein pistons are rotated by themselves under an arrangement in which the axial extending grooves formed along the outer circumferential surface on the slipper side end of each piston are applied to operate as a vane.

CONSTITUTION: A plurality of axial extending grooves 21 are formed circumferentially at an equal interval around the outer circumferential surface on the end of slipper 17 side of each piston 16, the sectional shape of the grooves 21 is formed like a saw-tooth pattern, projections 22 between the grooves 21 are formed to slant in the same circumferential direction so as to intensify the self-rotation of the piston 16. Under this arrangement, lubricating fluid is applied to the entire circumferences of the pistons 16 due to the self-rotation of the piston 16, while each of the grooves 21 causes the lubricating fluid to flow into a space between the piston 16 and a cylinder 10 so as to improve a lubricating characteristic between the piston 16 and the cylinder 10. Therefore, even in such an operating range as a small volume of oil is discharged, a good lubricating characteristic can be kept and the operation of the motor can be performed for a long period of time.