To provide a method of treating organic wastewater using an ultraviolet light of 185-254 nm in wavelength which can treat sludge efficiently at a low cost while saving energy and reduce the amount of excess sludge, and an apparatus for the method.

In the method of cleaning organic sewage by biological treatment, at least a part of sludge generated in a biological treatment process (activated sludge treatment process 1) for treating the organic wastewater 10 biologically is supplied to an ultraviolet radiation device 24 with a low- pressure mercury lamp installed and irradiated with the ultraviolet light of 185-254 nm in wavelength for a given time, and the treated sludge is returned to the biological treatment process (activated sludge treatment process 1) to be digested by microorganisms.