To reduce the discharge quantity of ash from desulfurizing equipment and furthermore, promote effective utilization of limestone and dolomite resources by using crude concrete sludge as desulfulizing agent in a furnace.

This desulfulizing method is a process of employing excessive components or the like of crude concrete to be used in construction, civil engineering industries as desulfulizing agent of a fluidized bed furnace and removes sulfur components in a gas generated in a normal pressure fluidized bed combustion boiler, a fluidized bed gasifying furnace or a fluidizing bed partially gasifying furnace. In this method, any of crude concrete sludge, dehydrated sludge cake and supernatant water is applicable. The crude concrete sludge is so arranged preferably in the fluidized bed furnace that Ca components in the crude concrete reaches 1-3 in molar ratio with respect to sulfur components in a fuel. The temperature of combustion and gasification in a normal pressure fluidized bed combustion boiler or the like is preferably within a range of 500-2,000°C and, moreover, for higher efficiency of desulfurization, it is set in a range 750-1,000°C to improve reaction rate.