A fluidized bed gasification furnace has a fluidized bed formed and a freeboard section F above a bed material so as to produce a produced gas by a reaction of raw material with a gasifying agent and take out the produced gas through the freeboard section. Upon shutdown of the furnace, inert gas is fed from below to the furnace for purging of gas in the fluidized bed with inert gas while steam is supplied to a freeboard section above the fluidized bed to purge a gas in the freeboard section with steam.