To provide a method and an apparatus for treating waste water by which water to be treated and malodorous gas are efficiently treated and the initial cost and the running cost are reduced.

In the apparatus and the method of treating waste water, the waste water containing amino ethanol and dimethyl sulfoxide is introduced into a denitrification vessel 1 provided in a 1st waste water treating apparatus 28, the malodorous gas produced in the 1st waste water treating apparatus 28 is decomposed in a 2nd waste water treating apparatus 29 of a separated line carrying out a separated waste water treatment, the 2nd waste water treating apparatus 29 for decomposing the malodorous gas has a 2nd nitrification vessel 20 having a semi-anaerobic part 25 and the malodorous gas introduced into a scrubber 11 from the 1st waste water treating apparatus 28 is treated with sludge circulated between the 2nd nitrification vessel 20 and the scrubber 11.