A gas turbine system comprising: a plurality of gas turbine engines (21, 22, 23) that are configured to be mounted in an airframe of an aircraft; generators (41, 42, 43) that are respectively connected to rotary shafts of the plurality of gas turbine engines (21, 22, 23); a battery that stores electricity generated by the generators (41, 42, 43); an electric motor that is driven by means of at least one of electricity from the generators (41, 42, 43) and electricity from the battery; a propeller that is driven by the electric motor; and a controller that is configured to control outputs of the gas turbine engines (21, 22, 23), wherein the controller is configured to decide at least one main starting gas turbine engine from the plurality of gas turbine engines before the plurality of gas turbine engines (21, 22, 23) are started, and to start the main starting gas turbine engine using electricity from the battery, characterized in that the controller is further configured to start a secondary starting gas turbine engine (22, 23) of the plurality of gas turbine engines other than the main starting gas turbine engine using only electricity from the generator connected to the main starting gas turbine engine after the main starting gas turbine engine reaches a steady state, to decide at least one main stopping gas turbine engine from the plurality of gas turbine engines before the plurality of gas turbine engines (21, 22, 23) are stopped, to stop driving of a secondary stopping gas turbine engine (22, 23) of the plurality of gas turbine engines other than the main stopping gas turbine engine, to perform motoring of the secondary stopping gas turbine engine (22, 23) using only electricity from the generator connected to the main stopping gas turbine engine, and to stop driving of the main stopping gas turbine engine and perform motoring of the main stopping gas turbine engine using electricity from the battery after motoring of the secondary stopping gas turbine engine (22, 23) is completed. a plurality of gas turbine engines (21, 22, 23) that are configured to be mounted in an airframe of an aircraft; generators (41, 42, 43) that are respectively connected to rotary shafts of the plurality of gas turbine engines (21, 22, 23); a battery that stores electricity generated by the generators (41, 42, 43); an electric motor that is driven by means of at least one of electricity from the generators (41, 42, 43) and electricity from the battery; a propeller that is driven by the electric motor; and a controller that is configured to control outputs of the gas turbine engines (21, 22, 23), wherein the controller is configured to decide at least one main starting gas turbine engine from the plurality of gas turbine engines before the plurality of gas turbine engines (21, 22, 23) are started, and to start the main starting gas turbine engine using electricity from the battery, to decide at least one main starting gas turbine engine from the plurality of gas turbine engines before the plurality of gas turbine engines (21, 22, 23) are started, and to start the main starting gas turbine engine using electricity from the battery, to start a secondary starting gas turbine engine (22, 23) of the plurality of gas turbine engines other than the main starting gas turbine engine using only electricity from the generator connected to the main starting gas turbine engine after the main starting gas turbine engine reaches a steady state, to decide at least one main stopping gas turbine engine from the plurality of gas turbine engines before the plurality of gas turbine engines (21, 22, 23) are stopped, to stop driving of a secondary stopping gas turbine engine (22, 23) of the plurality of gas turbine engines other than the main stopping gas turbine engine, to perform motoring of the secondary stopping gas turbine engine (22, 23) using only electricity from the generator connected to the main stopping gas turbine engine, and to stop driving of the main stopping gas turbine engine and perform motoring of the main stopping gas turbine engine using electricity from the battery after motoring of the secondary stopping gas turbine engine (22, 23) is completed. to stop driving of a secondary stopping gas turbine engine (22, 23) of the plurality of gas turbine engines other than the main stopping gas turbine engine, to perform motoring of the secondary stopping gas turbine engine (22, 23) using only electricity from the generator connected to the main stopping gas turbine engine, and to stop driving of the main stopping gas turbine engine and perform motoring of the main stopping gas turbine engine using electricity from the battery after motoring of the secondary stopping gas turbine engine (22, 23) is completed.