To reduce pressure fluctuations generated in a water fed part by judging a water usage amount have been changed to eliminate a checking time of pressure, when the rpm of a pump monotonously decreases or monotonously decreased between a prescribed range from the maximum rpm to the minimum rpm.

When practicing on a feed water device wherein three pumps 4 are operated in parallel, an arithmetic section in a controlling device 12 is allowed to memorize an arbitrary rpm which is less than the maximum rpm and more than the minimum rpm of the pump 4 starting first, and an arbitrary rpm (a first rpm) which is less than the maximum rpm and more than the minimum rpm of the pump 4 starting second. After starting of the pump 4 which should start first, or when the pump rpm is monotonously increased to the maximum rpm of the pump 4 owing to the increase of a water usage amount, except for the pump 4 starting last, the pump 4 operated at a stable speed not more than the first rpm immediately starts the pump 4 of the next starting turn.