The invention provides a method for treating waste water wherein organic and inorganic substances in waste water can be treated at high levels regardless of the concentration, and a unit itself can be made compact. The method of the invention is a method for treating waste water, which comprises treating waste water at 100.degree. C. or less in the presence of oxygen and catalyst containing noble metal and active carbon and having pores, wherein the volume of pores having a radius of 40 .ANG. or more and less than 100 .ANG. is 0.05 ml/g or more.