In a method for operating a combustor 10 of an embodiment, before ignition in the combustor 10, a mixed gas containing oxygen is circulated through the combustor 10 as a circulating gas. Then, in an operating time from the time of ignition in the combustor 10 to the time of a rated load of a turbine, from the time of ignition until reaching stable combustion conditions allowing stable combustion, a combustion gas in which a controller 100 controls a flow rate of a fuel supplied from a fuel supply part 40 and a flow rate of an oxidant supplied from an oxidant supply part 50 to maintain the same oxygen concentration as an oxygen concentration in the mixed gas is circulated as the circulating gas.