PURPOSE: To improve denitration efficiency, to save energy, to effectively remove SO3 and consequently to prevent the deposition of ammonium salts by dry- desulfurizing the waste gas with a carbonaceous adsorbent and then catalytically denitrating the waste gas.

CONSTITUTION: The waste gas at 80-130°C is passed through a dry desulfurizer 21, hence the temp. is hardly lowered, the components of SO3, Na, K, Cl, F, etc., are simultaneously removed, and dust is also removed. The waste gas passing the desulfurizer 21 is heated, added with ammonia and denitrated in a catalytic denitrator 60. Consequently, the loss of energy is reduced, the setting space is diminished, and the cost is reduced. Meanwhile, the trace metal elements in the waste gas are also effectively removed, hence the system is not adversely affected, and the deposition of ammonium salts in the system is prevented since SO3 is effectively removed.