To inexpensively and rapidly treat even waste water containing an antiseptic, agrichemials, a halogen-containing org. solvent or a surfactant without generating secondary waste or a new harmful substance.

By biologically treating waste water containing an antiseptic, agrichemicals, a halogen-containing org. solvent or a surfactant with activated sludge, a substance capable of being decomposed by activated sludge among substances to be treated in waste water is decomposed at first and, by subsequently applying photochemical treatment to waste by using a photocatalyst such as titanium oxide, the substance incapable of being decomposed by activated sludge is decomposed or converted to a biologically decomposable structure and, next, waste water is further biologically treated with activated sludge to certainly decompose almost all of substances to be treated in waste water under a mild condition.