NOVELTY - Nitrogen processing of wastewater by switching operation which introduces the obtained high temperature heating liquid mixture into the denitrifying process, comprises: (a) nitrous-acid-formation process of oxidizing reduced nitrogen compound of wastewater to nitrous acid by activated-sludge liquid mixture; (b) nitrogen removal of wastewater; and (c) heat-processing activated-sludge liquid mixture of the nitrous-acid-formation process at 40-55 degrees C.    USE - The process or apparatus is useful for nitrogen processing of wastewater (claimed).    ADVANTAGE - The structure of processing apparatus can be simplified. Replenishment of the hydrogen donor is easy and economical.    DETAILED DESCRIPTION - Nitrogen processing of wastewater by switching operation which introduces the obtained high temperature heating liquid mixture into the denitrifying process, comprises: (a) nitrous-acid-formation process of oxidizing reduced nitrogen compound of wastewater to nitrous acid by activated-sludge liquid mixture; (b) nitrogen removal of wastewater, each process of denitrification process of carrying out reduction of nitrous acid, and the heat treatment process which heat-processes the activated-sludge liquid mixture on anoxia conditions in the heat treatment process; and (c) heat-processing activated-sludge liquid mixture of the nitrous-acid-formation process at 40-55 degrees C for making a nitrous-acid-oxidation microbe selectively deactivate, and medium temperature heating-mixing liquid is obtained, where operation which introduces the obtained medium-temperature heating mixing liquid into the denitrifying process, it heat-processes on the high temperature conditions 60 degrees C or more for reforming the activated-sludge liquid mixture of the nitrous-acid formation process to the hydrogen donor of denitrification, and a high temperature heating liquid mixture is obtained. An INDEPENDENT CLAIM is an apparatus for nitrogen processing of wastewater by having an outflow path for flowing heat-processed liquid mixture out into the denitrification means, comprising: (a) a nitrous-acid-formation means to oxidize the reduced nitrogen compound of a wastewater to nitrous acid by an activated-sludge liquid mixture; (b) denitrification means which carries out reduction denitrification of the nitrous acid; (c) nitrogen removal apparatus of a wastewater which has each means of a heat processing means to heat-process the activated-sludge liquid mixture on anoxia conditions, where the inflow path for flowing in the activated-sludge liquid mixture from the nitrous-acid-formation means into the heat processing means; and (d) heating means for switching the heat treatment condition of the activated sludge liquid mixture which flowed in to the 40-55 degrees C medium-temperature conditions for making a nitrous-acidoxidation microbe selectively deactivate, and the high temperature conditions 60 degrees C or more for reforming to the hydrogen donor of denitrification by turns, and heat-processing.