A function of absorbing CO2 in gas by chemical to accelerate gasifying reaction is made compatible with a catalytic function of reforming tar in gasified gas generated by the gasifying reaction to thereby make it possible to produce clean product gas with high gasification efficiency.Gasification process is divided into three processes: a gasification furnace 10 for carrying out gasification process by pyrolysis and gasification (pyrolysis gasification phase, first process), a combustion furnace 20 for burning char to obtain calcined active chemical (char combustion phase, second process) and a gas purification furnace 30 for purifying gasified gas (gasified gas purification phase, third process). Through heat transmission by fluid heat medium and chemical and through harmony of chemical reactions in respective phases by the chemical, the gasification furnace 10 is independently controlled to a low or medium temperature (773-1073° K) which is required for gasification and which enables absorption of CO2; and the gas purification furnace 30 is controlled to a high temperature (1073° K or more) required for gas purification.