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 DEG 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 DEG K or more) required for gas purification.