To miniaturize a device for treating liquid with ultraviolet rays by making the heat radiation means of a ballast for lighting a ultraviolet lamp unnecessary or by reducing its dimensions, and to miniaturize a power supply box by arranging the ballast so as to be separated from the power supply box.

This device for treating liquid with ultraviolet rays is provided with a pipe line 1, through which a liquid-to-be-treated P passes, a translucent lamp protecting tube 4 disposed in the pipe line and contacted with the liquid-to- be-treated P, and an ultraviolet lamp 5 housed in the lamp protecting tube 4. The ballast 8 for the ultraviolet lamp is arranged in close vicinity to the outer surface of the pipe line 1 so that heat can be radiated to the pipe line 1. Cooling effect by the liquid-to-be-treated realizes the efficient heat radiation of the ballast 8. A cooling medium C (for example, a cooling liquid) may be put between the outer surface of the pipe line 1 and the ballast 8.