Unburned carbon is efficiently separated according to the properties of fly ash. An apparatus is provided which comprises: a slurry preparation tank (10) in which water (c) is added to fly ash (a) to produce a slurry (d); a scavenger addition device (20) for adding a scavenger (e) to the slurry (d); a vertical surface-modification device (30) in which the slurry after addition of the scavenger is stirred at a high speed to impart shear force thereto and thereby cause the scavenger (e) to adhere to the surface of unburned carbon (b) contained in the slurry; a regulating tank (60) in which a blowing agent (f) is added to the slurry (d′) which has undergone surface modification with the surface modification device; and a flotation machine (70) with which the unburned carbon (b) is floated together with bubbles (n) and separated from the slurry containing the blowing agent.