Abstract: An ammonia injection device for use in an exhaust gas denitration system comprises ammonia injection pipe units arranged in a multiplicity of stages within an exhaust gas duct and each comprising a horizontal header pipe and a plurality of ammonia injection pipes extending downward from and connected to the header pipe and each having a multiplicity of injection nozzles. The header pipes other than the uppermost header pipe and a guide fixing member disposed horizontally at a bottom portion of the duct are each provided with a plurality of upward tubular guides arranged at a predetermined interval. The injection pipes have their lower ends inserted in the respective guides upwardly and downwardly movably. The injection nozzles have an opening diameter gradually increasing as the injection pipe extends downward.