Disclosed is a method for removing unburned carbon from fly ash at low cost and within a short time. The method comprises the steps of adding a collecting agent to fly ash directly, agitating/mixing the mixture in a mixer (5), adding water to the resulting mixed material in a mixing vessel (7) to yield a slurry, applying a shearing force to the slurry in a submerged stirrer (9), and performing flotation separation of unburned carbon in a flotator (15).