PURPOSE: To improve thermal efficiency by desulfurizing combustion gas generated in a gasification combustor by a limestone desulfurization device at a high temperature.

CONSTITUTION: In a gas cooler 31, thermal exchange is carried out between combustion gas generated in a gasification combustor 19 and fed through a duct 11 and steam is generated in an exhaust gas boiler 19 and fed through a duct 35. The temperature therein is decreased to a value required for high- temperature desulfurization, that is, temperature between 900°C and 1100°C. The resultant gas is fed to a high temperature limestone desulfurization device 32. Limestone given by a line 36 is charged in a fluid layer form in the device, and high desulfurization efficiency more than 95% is obtained through the contact of the resultant gas and the limestone. Alkaline metal such as V or Na is simultaneously removed by the limestone. Careful cleaning of the gas is then performed by a dry dust remover such as a cyclone separator 33 or a porous filter 34. Afterward, the gas is supplied to a gas turbine combustor 14 and burned. The high temperature steam generated in the gas cooler 31 is supplied through a duct 37 to a steam turbine 21.