PURPOSE: To efficiently preheat a fluidized bed in a short time by firstly fluidizing a fluidization medium by high-temp. air to heat the fluidized bed at the prescribed temp. and then supplying solid fuel having low ignition temp. into the fluidized bed and burning it.

CONSTITUTION: In the case of fluidizing a fluidization medium 100 (e.g. alumina particles) and preheating a fluidized bed F, the fluidization medium 100 is firstly fluidized by high-temp. air via an air header 31, an air pipe 30 and an air dispersion pipe 40 and the fluidized bed F is heated at the prescribed temp. Then, this fluidized bed F is furthermore heated by supplying solid fuel (e.g. polyethylene solid fuel) having low ignition temp. into the fluidization bed F from a supply port 112a and burning it and the fluidization bed F is preheated. As a result, the fluidized bed is efficiently preheated in a short time and normal operation is started in an early period.