To provide a method and an apparatus for treating waste water capable of efficiently processing waste water containing dioxane at hundreds of mg/L high concentrations for decreasing the concentrations to several mg/L.

The apparatus for treating waste water 10 includes: a first reaction tank 12 in which waste water containing dioxane is injected; an ozone generator 16 for supplying ozone gas to the first reaction tank 12; an alkali agent tank 18 for supplying an alkali agent to the first reaction tank 12; a second reaction tank 14 in which the waste water treated by the first reaction tank 12 is injected; an ultraviolet irradiation apparatus which is placed in the second reaction tank 14; and a waste ozone tube 20 for supplying ozone gas to the second reaction tank 14.