To provide a light irradiation device in which the soil stuck to a light transmissible tube is efficiently removed and high frequency power is efficiently inputted to an electrodeless discharge lamp and a water treating device.

The light irradiation device is provided with a cleaner 4 for removing the sticking material stuck to the outside surface of the light transmissible tube 3 by bring the cleaner 4 into contact with the outside surface of the light transmissible tube 3 and a moving means for the cleaner 4, which is formed so as to reciprocally and rotationally move the cleaner 4 along the outside surface of the light transmissible tube 3.