Abstract: To sterilize and purify continuously water in a water tank, a water treating apparatus is provided that comprises a circulating type water pipe for taking the water in the water tank outside the water tank and then for returning the water into the water tank, and an ozone supply apparatus, a filtering apparatus, and an ultraviolet-light irradiating apparatus that are situated in the course of the water pipe, so that water can be sterilized and purified thoroughly by a three-stage treatment incluing "an ozone treatment" wherein the decomposition of organic substances, deodorization and inactivation of microorganisms by ozone can be expected, "a filtering treatment" wherein the elimination of foreign matters and microorganisms can be expected, "an ultraviolet treatment" wherein the sterilization by ultraviolet rays can be expected.