PURPOSE: To enhance desulfurization efficiency by injecting only water into generated exhaust gas without freshly adding an alkali agent thereinto an humidifying it within an unsaturated range of water and allowing sulfur compd. incorporated in the exhaust gas to react with the alkali regent.

CONSTITUTION: Nonreacted quick lime and gaseous sulfur dioxide besides coal ashes are contained in exhaust gas discharged from a combustion furnace 1 and heat-exchanged with air for combustion in a gas-air heater 2. The reaction of quick lime and gaseous sulfur dioxide is promoted by making the partial pressure of steam high in a range not more than saturated vapor pressure by means of water in a spray cooler 3 and making the temp. low and further dust contg. quick lime is collected with a dust collector 4 provided to the rear flow. In this case, gaseous sulfur dioxide is adsorbed to and absorbed by the dust layer and remaining sulfur dioxide which has been not desulfurized in the furnace 1 is removed and the exhaust gas is discharged to the atmosphere through a stack 5.