PURPOSE: To obtain by means of a simple structure, a stable operating state wherein no cavitation is generated, by avoiding the complicate adjustment for changing the operating condition of a pump and the operation for increasing the minimum flow rate more than required.

CONSTITUTION: A pump is provided with an outlet 5A formed on a discharge casing 3B, an injection hole 5B formed on a suction casing 3A, and a communication pipe 5C for communicating the outlet 5A with the inlet of the injection hole 5B, and an injection passage 5 wherein the outlet of the injection hole 5b is pointed and opened to the low pressure part 2A of an impeller inlet is provided, and a cooling means 6 is provided in the communication pipe 5C in this injection passage 5, and discharge liquid discharged from the impeller 3 to the discharge casing 3B is partially cooled on the way of the communication pipe 5C by the cooling means 6, and then it is injected to the low pressure part 2A of the impeller inlet.