PURPOSE: To achieve the high flow by providing a centrifugal pump at the upstream of a reproduction pump while conducting each sucking section through the bypath and making the flow-lift curve sharp under low flow while making gentle under high flow.

CONSTITUTION: Under such low flow as coming in the operating range of a centrifugal pump having an impeller 11, the water will flow into the reproduction type impeller 8 to produce the high pressure water and the check valve 16 will close the outlet. Consequently the flow-lift characteristic will have the sharp inclination as shown in the range A of the drawing. When the flow will exceed over the limit flow of the centrifugal pump, the water can not be fed to the reproduction type impeller 8 thereby the pressure at the sucking section of the reproduction type impeller 8 will be lower than that at the sucking section of the centrifugal pump thus to open the check valve 17 and the water will flow directly into the reproduction type impeller 8 to continue the pumping function thus to make the inclination of the flow-lift curve gentle resulting in the high flow.