PURPOSE: To enhance the UV radiation efficiency and to prevent the generation of reddish water by providing an electrodeless lamp while preventing direct contact with a fluid to be treated, and projecting the reflected light or condensed light of the UV from the electrodeless lamp on the fluid.

CONSTITUTION: A transparent water supplying and distributing pipe 11 is allowed to pierce a main body 10 consisting of a closed columnar or elliptic cylindrical box body, and connected with a water supplying and distributing pipe 5 to form a water sterilizing device 4. The electrodeless lamp 12 receiving a microwave and emitting UV rays is fixed at a specified position in the main body 10. A metallic mesh 13 for shielding microwaves is provided between the lamp 12 and the water supplying and distributing pipe 11, and a reflecting member 14 for condensing the UV rays radiated from the lamp 12 on the transparent supplying and distributing pipe 11 is furnished on the inner side surface of the main body 10. Accordingly, since UV rays can be radiated without bringing the electrodeless lamp 12 into contact with feed water, the lamp 12 can be started in a short time, and the UV radiation efficiency is also improved.