PURPOSE: To efficiently pressurize water with compressed air and improve the water supply performance by connecting an air relief valve and a pressure regulator of the compressed air on the upper section of a pressure tank and connecting a pump interlocked with a water quantity detecting sensor on the lower section.

CONSTITUTION: Compressed air 3 and water 2 are filled at the upper section and the lower section respectively in a pressure tank 1. A water pressure-feeding pump 4 is connected to the lower section of the pressure tank 1 via a check valve 5. An air relief valve 10 operated by a pressure switch 7a is connected to the upper section of the pressure tank 1. An air pipe 9 is connected to the upper section of the pressure tank 1 via a pressure regulator 11, a water quantity detecting sensor 13 is connected. The water quantity in the pressure tank 1 is properly maintained by the pump 4 interlocked with the water quantity detecting sensor 13, water 2 is pressurized by the proper compressed air 3 from the air pipe 9, a large quantity of water is fed through a water supply pipe 6.