PURPOSE: To eliminate the wasteful operation power and the fan noise and reduce the operation time by providing a discharge pressure sensor and a pressure tank in a column pipe connected to the discharge side of a pump through a non-return valve, and operating a cooling fan for a water-supplying device only when the inverter output is present.

CONSTITUTION: A cooling fan 15 is connected to a primary side circuit C1 of an inverter 11, from a circuit C3 through a switch 16. A comparator 17 connected to the inverter output side C2 from a circuit C4 compares the output of the inverter 11, i.e., the output frequency or the operation frequency with the preset frequency by a setter 18. When the operation (output) frequency> the preset frequency, i.e., when a pump 3 is operated, the switch 16 is closed and the cooling fan 15 is operated. On the other hand, when the pump 3 is stopped, the switch 16 is opened and the cooling fan 15 is operated, and the pump 3 is interlocked with the cooling fan 15. Thus, the waste of the power that only the cooling fan is operated when the water-supplying device is stopped is avoided, and the life of the cooling fan is improved.