PURPOSE: To prevent falling or dislocation of shoes without receiving springs in cylinder holes by providing springs to energize the shoes toward a stator in the space between the outer circumference and the stator, and supporting the end parts with the outer circumference of a rotor.

CONSTITUTION: A supporting member 30 is made of belt-like metallic plate, an assembling body 60 in which a shoe 35 and a piston 25 are assembled in the supporting member 30, is positioned in a space B partitioned off by a rotor 20 and the inner race 37 of a ball bearing, and the shoes 35 are energized to be brought in contact with the inner circumference of the inner race of the ball bearing by spring function due to engagement of the side plate parts 30A, 30C of the supporting members 30 with the rotor 20. Hereby even when a motor 3 is not operated, the shoes 35 can be pressedly contacted with the inner race of the ball bearing. Consequently, even if vibration is applied from the outward when the pump device is not operated, falling or dislocation of the shoes from the pistons can be prevented.