To reduce the maintenance cost and to improve the efficiency of sterilization, etc., by the irradiation with ultraviolet rays by installing electrodeless discharge lamps excited by high frequency, etc., in which mercury, etc., are sealed and controlling the number of the lit discharge lamps in response to the change of the flow rate of liquid to be treated.

A liquid to be treated such as sewage is introduced into a treatment tank 5, irradiated with ultraviolet rays from electrodeless discharge lamps 1 to be sterilized, made to overflow from a flow rate measuring dam 6, and introduced into a storage tank 9. When the amount of sewage to be introduced is decreased, and the water level of the tank 5 is lowered, the level is detected by a level detector 7, and a detection signal is output to a flash controller 8. An extinction signal is output from the controller 8 to high frequency generators 4 the number of which corresponds to the variation of the level, and concerned lamps 1 are extinguished. In this way, the number of lit lamps 1 is controlled in response to the change of the flow rate of sewage to optimize sewage treatment constantly.