PURPOSE: To eliminate labor for washing and to improve productivity by maintaining the inside of a storage tank in which the produced ultra-pure water is stored in the pressurized state at all times by the gas sealed therein through a germ-free air filter at all times.

CONSTITUTION: The ultra-pure water produced with a device 1 for producing ultra-pure water is stored in a storage tank 2. A germ-free air filter is provided by a vent pipe 2' at the top of the tank 2 and a pressure switch 6 is provided in a suitable position. The inside of the storage tank is maintained in the pressurized state at all times by the gas sealed through the germ-free air filter by which the backflow of bacteria from a place 4 of use into the pipeline is prevented. The washing operation is thus considerably decreased and productivity is improved.