To enable miniaturization of a housing, the light weight, and the increase of the stroke of a piston.

A pump case 20 is formed by being divided into a front half part 20-1 and a rear half part 20-2, a reservoir 16 is provided in the pump case front half part, and a ring-shaped recessed part 40 whose front part is opened is formed in the pump case rear half part and around an arranging space of an output shaft 12a of a motor. A facing ring-shaped flexible oil bag 42 is arranged and fixed by communicating the inside with the reservoir 16 in the ring-shaped recessed part with close-contact of the opening end to the back surface of the pump case front half part 20-1.