To stabilize operation while saving labor by realizing such a control that the gasification temperature and the height of the fluidized bed are readily maintained constant, even if the properties and the shape of a combustible matter such as refuse are nonuniform.

Detecting the gasification temperature of a gasification furnace 1 by a gasification temperature detecting means 12 and detecting the height of a fluidized bed 3 by a fluidized bed height detecting means 13, control means 14 controls, while monitoring the gasification temperature and the height of the fluidized bed, the feed rate of a combustible matter such as refuse by a combustible matter feeding device 10, the discharge rate of furnace-bottom ash by a furnace-bottom ash discharging device 11 and the feed rate of gasification air by a flow control valve 8, three essential elements that affect the gasification temperature and the height of the fluidized bed, so that the gasification temperature and the height of the fluidized bed may be maintained at set points.