PURPOSE: To reduce a load loss by connecting a displacement type rotary pump to a drive motor through a one-way clutch and a suction port of a centrifugal pump to communicate with a suction port of the displacement type rotary pump through a bypass having a check valve.

CONSTITUTION: A drive motor 1 connects a centrifugal pump 10 to one end and a gear pump 16 to the other end through a one-way clutch 11. This drive motor 1 connects a control part which normally and reversely inverts the drive motor 1, and the centrifugal pump 10 forms in its suction port 6 a bypass port 20 providing a check valve 19. This bypass port 20 communicates with a suction port 14 of the gear pump 16 through a bypass pipe 21. In this way, when the drive motor 1 is inverted, for instance, in a normal direction of rotation in the point of time completing the priming, because the gear pump 16 is released from its power transmission by action of the one-way clutch 11, the gear pump 16 improves its durability while enabling the drive motor 1 to reduce its load loss.