PURPOSE: To adjust largely a control area for water head and flow rate characteristics of a pump by forming a selective flow conduit which acts to communicate selectively a suction side flow conduit with a discharge side flow conduit on the connecting head of an auxiliary tank.

CONSTITUTION: A plurality of suction side flow conduits 48, 50 and ejection side flow conduits 49, 51 are formed continuously with a pump causing and an auxiliary tank 38. And selective flow conduits 52, 53 for communicating selectively the suction side flow conduits 48, 50 and ejection side flow conduit 49, 51 with the connecting head 45 of the auxiliary tank 38 are also formed. The connecting condition of an impeller is switched into series or parallel by switching the connecting condition of the connecting head 45. In this manner, the control area for the water head and flow rate characteristics can be largely adjusted to meet various types of demand.