PURPOSE: To maintain the dose of short wavelength ultraviolet rays at high output level by controlling voltage and intensity of current to be supplied to a low pressure ultraviolet ray lamp basing on the detected temp. of the water to be treated at the lower temp. side than the peak value of the dose of a short wavelength ultraviolet ray lamp of low pressure.

CONSTITUTION: A temp. detector 5 is provided to the inside of a treating tank 1 and connected to a temp. detecting unit 6 provided to the outside of the tank 1. The temp. in the treating tank 1 detected by the temp. detector 5 is used as the control signal for the temp. detecting unit 6 which is added to a dimmer device 7 provided to a low pressure mercury lamp 2. The voltage or intensity of current to be supplied to the low pressure mercury lamp 2 is controlled in the dimmer device 7 by the above described signal and is supplied to a lighting device 8 of the low pressure mercury lamp 2. The waste to be treated is fed at ca. 10W40°C water temp. to the treating tank 1 in the above described water treating apparatus. On one hand, the operation of the low pressure mercury lamp 2 is preferred to be set to result 20W30°C temp. as optimum temp.