PURPOSE: To suppress wear of a vacuum pump and thereby eliminate wasteful use of the power by keeping away the air with a vacuum pump, in case when the air existing in the centrifugal pump hinders pumping-up, and by shutting off the drive of vacuum pump after the centrifugal pump is put in operation.

CONSTITUTION: In case no water exists in the pump chamber 2 at the time of starting the pump, the revolving drive shaft 4 rotates the rotor 6 of the vacuum pump 5 to permit the blade plate 16 to rotate incontact with the internal surface of the casing 17, which will suck the air out of the pump chamber 2 through a communication pipe 14 into the flowout pipe 12, whereby inside of the pump chamber 2 will become in negative pressure to pump up the water for filling the pump chamber 2. Then the centrifugal pump 1 is put in operation to suck up the water, which flows out to the flowout pipe 12 through a check valve 13. Thus the water pressure in the pump chamber 2 rises, and this water at high pressure is sent to the clutch chamber 7 through a communication pipe 10 to push the clutch 8 in the direction of disengagement, which will decouple the drive shaft 4 to stop the rotor 6 and thereby the vacuum pump 5 also, so that wear of the vacuum pump 5 can be suppressed and wasteful use of the power, eliminated.