A method for treating waste water containing nitrate ion which comprises supplying raw waste water from a tube (2), formalin from a tube (3) and an aqueous alkaline solution from a tube (4) to a mixing vessel (1), while agitating with an agitator (5), so the resultant mixture has a pH of 7 or higher, transferring the mixture from an outlet pipe (6) to a catalyst bed (9) through a tube (8) by a pump (7), and discharging the treated fluid drained from the catalyst bed (9) to the outside of the system through pipe (11), wherein the catalyst bed (9) is provided within a jacket (10), through which a liquid such as water flows, for the temperature control, and is packed with a support type catalyst comprising spherical type activated carbon and Pd—Cu supported thereon. The type of reactor containing the support type catalyst in the catalyst bed (9) may be a fluidized bed or a fixed bed. The above method can be suitably used for reducing nitrate ion to nitrogen by the use of formalin as a reducing agent at a low treating cost and with a low selectivity of ammonia as a by-product.