A decreasing system of nitrogen oxide is provided to offer safety from danger of toxic chemicals, and to maximize economical availability by using high pressure and high temperature air with urea in a selective catalytic reduction method. A decreasing system of nitrogen oxide includes the followings: a reservoir(301) storing urea into urea water; a reorganization supply module(302) supplying urea water of the reservoir according to discharge amount of the nitrogen oxide; a minute jet(314) spraying the urea water to size of 50 ~ 100 mum; a compressed air offering part(320) supplying compressed air to facilitate the urea water; a decomposition bath(303) providing ammonia by supplying decomposition reaction on the urea water; a heated air offer part(304) providing the air of 250~500‹C; an ammonia injection facility(306) generating ammonia; a catalytic reaction tower(307) reducing the nitric oxide; and a preheater(308) preheating the vapor.
