To provide a method of early recovering an in-furnace fluidized state of a fluidized bed boiler capable of early recovering a good state when a kind of coal is switched and the deteriorated in-furnace fluidized state of the fluidized bed boiler is recovered to the good state.

In the fluidized bed boilers 10, 20 in which layer heights in furnaces 11, 21 are adjusted by supplying bed materials stored in bed material tanks 31, 32, 41, 42 to the furnaces 11, 21, or extracting the bed materials in the furnaces 11, 21 to the bed material tanks 31, 32, 41, 42, the bed materials in the furnaces 11, 21 and the bed materials of the bed material tanks 31, 32, 41, 42 are positively replaced when the deteriorated in-furnace fluidized state is recovered to the good state by switching the kind of coal.