To provide a fuel cell system equipped with a compact and low-noise compressor capable of carrying out high-efficiency operation with little energy consumption at normal operation, and capable of a high-output operation in a high-load, low-pressure and low-temperature environment.

The fuel cell system is provided with a first flow channel 29 in which oxidant gas circulates an oxidant gas supply channel 11 from a compressor part 14a to a fuel cell 2, passes through an oxidant gas flow channel of the fuel cell 2 and is exhausted from an oxidant gas exhaust flow channel 12 to the atmosphere, a second flow channel 30 in which the oxidant gas passes through a turbine part 14b from a branched part S fitted to the oxidant gas supply channel 11 from the compressor part 14a to the fuel cell 2 and flows into an inflow part P fitted between the branched part S of the oxidant gas supply channel 11 and the fuel cell 2, and a control device 7 changing over between the first flow channel 29 and the second flow channel 30. The control device 7 carries out changeover between the first flow channel 29 and the second flow channel 30 in response to operation modes of the high-efficiency operation mode and the high-pressure operation mode.

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