The object is to remove unburned carbon in a fly ash in a stable and economically advantageous manner. A fly ash, water and a trapping agent are mixed together in a hybrid mixer (2), a shearing force is applied to the mixture to prepare a slurry containing surface-modified unburned carbon within a short time, a foaming agent is added to the slurry, and then the unburned carbon is separated by performing flotation separation in a flotator (11).