PURPOSE: To prevent an NH3 leak while high denitration ratio is kept, in a desulfurizing and denitrating method for passing exhaust gas through two reactors each filled with a carbonaceous catalyst while mixing NH3 in said gas, by adding SO3 or H2SO4 to the outlet gas of the second reactor before dust collection.

CONSTITUTION: NH3 is added to NOx and SOx-containing exhaust gas from a line 2 while the NH3 added gas is introduced into a moving bed reactor 3 receiving a carbonaceous catalyst 4 to perform desulfurization and denitration. NH3 is again injected into the gas exhausted from the reactor 3 from a line 6 while the NH3 added gas is passed through a moving bed reactor 7 receiving a carbonaceous catalyst 8 to perform denitration and desulfurization and, thereafter, SO3 or H2SO4-containing gas is introduced into a line 9 to convert leaked NH3 in the gas to (NH4)2SO4 or NH4HSO4 which is, in turn, collected along with fly ash by a dust collector 10. The catalysts discharged from the reactors 3, 7 are regenerated under heating in a regenerator 12 to be recirculated. The high concn. SO2 gas recovered from the regenerator 12 is used as a stock material of SO3 or H2SO4 to be added to the outlet gas of the reactor 7.