NOVELTY - Combined production of solid fuel and nitrogen oxide reducing agent comprises: producing a composite using an organic waste; producing bio-gas; supplying bio-gas to boiler; separating solid components and liquid components from the waste liquid; adding an element of liquefied fertilizer, which is the liquid object separated from waste liquid; contacting the fertilizer to combustion gas; naturally drying the solid component; injecting dried solids into a dryer; molding dried solids to produce a solid fuel; and introducing the solid fuel into a furnace for heating to produce electricity.    USE - The method is useful for combined production of solid fuel and nitrogen oxide reducing agent (claimed).    ADVANTAGE - The method is capable of performing organic waste treatment with improved process efficiency to provide high value-added by-products.    DETAILED DESCRIPTION - Combined production of solid fuel and nitrogen oxide reducing agent comprises: producing a composite using an organic waste; producing bio-gas by sequentially carrying out methane fermentation and acid fermentation for the organic waste slurry containing livestock excretion in an anaerobic digestion tank; supplying the generated bio-gas to a boiler to produce heat; separating solid components and liquid components from the waste liquid discharged from the anaerobic digestion tank; adding an element of liquefied fertilizer, which is the liquid phase object separated from the waste liquid; injecting the fertilizer into upper portion of a combustion chamber of a heat power plant or an incinerator; contacting the fertilizer to a combustion gas used as a nitrogen oxide reducing agent; naturally drying the moisture contained in the solid component separated from the waste liquid by supplying the solid component into a bio-dryer; injecting the dried solids into a dryer by mixing the agricultural by-products; molding the dried solids into a pellet form to produce a solid fuel; and introducing the solid fuel into a furnace for heating to produce electricity.