A method and device for controlling retention time of a fluid medium in a fluidized-bed gasification furnace in a gasification facility are provided. A fluid medium is guided to any of a plurality of fluid-medium charge ports connected to a gasification furnace at intervals from an upstream end thereof toward a downstream side in a distribution direction of a fluid medium longitudinally of the gasification furnace. A raw material is guided to any of a plurality of raw-material charge ports connected to the gasification furnace at positions longitudinally of the gasification furnace and correspondingly to the fluid-medium charge ports. The fluid medium is extracted and guided to a combustion furnace from a fluid-medium extraction port connected to a downstream end of the gasification furnace longitudinally thereof in the distribution direction of the fluid medium. Thus, retention time of the fluid medium in the gasification furnace is controlled.