PURPOSE: To change nitrogen oxide contained in a treating water into nitrogen gas by the action of active sludge by using an organic matter as a nutrition source and to improve the economical efficiency, by adding treating water from which the ammonia nitrogen has been removed, when the treating water is treated by denitration treatment after nitrification treatment.

CONSTITUTION: The solid matter contained in the treating water containing organic matters and ammonia nitrogen is removed at pretreating tank 1, and the treating water is treated by nitrification treatment and denitration treatment by the action of active sludge while the treating water is circulated in mixing-decomposing tank 2, and then the water is supplied to denitrating tank 3. On the other hand, a part of treating water from tank 1 is supplied to absorbing tower 4, and only ammonia nitrogen is removed from the treating water by the absorbing action of zeolite, and the water is supplied to denitrating tank 3. The nitrogen oxide remained in the treating water is treated and changed into nitrogen gas by the acton of active sludge utilizing the organic matter as the nitrition source.