To simply treat waste water containing nitrate nitrogen by a small- sized apparatus, to enable treatment stable against the fluctuations of load such as a change in an amount of nitrate nitrogen and to effectively treat even nitrate nitrogen high in concentration.

Hydrazine and divalent copper ions are added to water to be treated containing nitrate nitrogen and this waste water is brought into contact with a Raney-nickel catalyst or a ruthenium catalyst to reduce nitrate nitrogen. The addition amount of hydrazine is set to 3-4 time-equivalent with respect to nitrate nitrogen in waste water to be treated and the addition amount of copper ions is set to 0.2-3 mg/L as a concentration in waste water to be treated. The pH of waste water to be treated is preferably adjusted to 11.7 or more.