PURPOSE: To prevent water hammer by connecting a means of gradually lowering the output of a motor to the part between a power source and the motor winding if a pump and gradually bringing a negative pressure part which is generated on a feed water side at the time of ordinary rotation of an impeller close to atmospheric pressure.

CONSTITUTION: The motor winding 11 of a pump motor is connected to an AC power source 12 via a thyristor 13. The gate terminal of the thyristor 13 is controlled by a control means 14 consisting of a microcomputer and based on a voltage applying timing which is previously stored in a memory part 15. At this time, the period from the turn-on to the turn-off of the thyristor 13 is gradually reduced, gradually lowering the output of the pump motor. Thereby, at the time of the ordinary rotation of the impeller of a pump device, a negative pressure part which is generated on a feed water side can be gradually brought close to atmospheric pressure, relieving the backward flow of water to prevent the occurrence of water hammer.