Abstract: To reduce cost of chemical by reducing quantity of foaming agent used when removing unburned carbon in fly ash with wet floatation and to prevent decrease in activity index of the fly ash. In wet decarburization of fly ash, the solution comprising the steps of: adding water to fly ash to generate slurry; adding hydrophobizing agent and foaming agent to the slurry and agitating them to generate bubbles; and adhering unburned carbon in the fly ash to the bubbles to float them to remove the unburned carbon in the fly ash, when sedimentation component at wet floatation separation is solid-liquid separated and liquid phase obtained by solid-liquid separation is reused for another floatation separation for new fly ash, the quantity of foaming agent added to the slurry is adjusted such that concentration of the foaming agent in liquid phase is in a predetermined range.