Coal is gasified by a gasifier to produce coal gas, whose main constituents are carbon monoxide (CO) and hydrogen (H2). The coal gas has its heat recovered in a syngas cooler and its impurities and sulfur content removed by a gas clean-up unit. The gas is then burned by gas turbine combustors, to drive a gas turbine. The gas turbine’s combustion exhaust gas is released through a stack after having its heat recovered by a heat recovery steam generator. Meanwhile, steam produced with the heat recovered by the syngas cooler and the heat recovery steam generator is used to drive a steam turbine. Carrying out such combined power generation using both a gas turbine and a steam turbine enables higher generation efficiency to be achieved than with the conventional pulverized coal thermal power generation.