To provide the ultraviolet emission amount of ultraviolet rays generated by a discharge lamp sufficeint for sterilizing legionella.

A high voltage discharge lamp 31 is provided with the lamp power of 250W, and when the radius of a jacket 30 is set as 1.25cm and the inner radius of a sterilization water flowing pipe 33 is set as 2.5cm and the ultraviolet transmittance of the jacket 32 is set as 0.72, the ultraviolet strength is approximately 35mW second/cm2 on the inner face of the sterilization water flowing pipe 33, and 99.9% of legionella in hot water is killed by passing hot water in the sterilization water flowing pipe 33 just one time at the flow velocity of 10cm/second. Also the ultraviolet rays of 365nm and 435nm emitted from the high voltage discharge lamp 31 are brought into contact with a photocatalyst such as titanium oxide on the surface of the jecket 32 and the inner surface of the sterilization water flowing pipe 33 to generate electrons, and the hot water sterlization by the active electrons is performed.