PURPOSE: To prevent the decomposition of H2O2 and to enhance denitrating performance by constituting an injection pipe for H2O2 of a double pipeline and passing H2O2 through the inner pipeline and also allowing cooling water to flow through the outer pipeline and injecting NH3 upstream for the position to be injected with H2O2.

CONSTITUTION: After injecting NH3 into exhaust gas 1 contg. NOX at an upstream, H2O2 8 is atomized into a duct from nozzles 10 through an injection pipeline 9 constituted of a double pipeline having both an injection pipe 15 for H2O2 to the inside and a pipeline 16 for cooling water to the outside and thereby NOX is reduced in a vapor phase. In this case, since H2O2 is cooled, it is prevented from being decomposed into H2O and O2 before atomization and the thermal expansion of the injection pipeline 9 can be prevented.