A power generation system comprises a fuel gas supply device 13 for controlling methane concentration or carbon dioxide concentration in a mixed gas MG containing methane and carbon dioxide within a setting range for the concentration in the fuel gas of a gas engine 11, and for supplying the mixed gas MG to the gas engine 11 as the fuel gas, and a gas concentration sensor 14 for measuring the carbon dioxide concentration or the methane concentration of the mixed gas MG. The fuel gas supply device 13 comprises a carbon dioxide removal device 16 for removing carbon dioxide in the mixed gas MG, and an operating condition control device 17 for controlling an operating condition that affects an increase or decrease of a carbon dioxide removal rate of the carbon dioxide removal device 16, and the operating condition control device 17 controls the operating condition of the carbon dioxide removal device 16 based on the measurement result of the gas concentration sensor 14, thereby controlling the concentration of methane and carbon dioxide in the mixed gas.