A fuel cell system comprising: a plurality of fuel cell stacks; fuel gas supply paths configured to supply fuel gas to the respective plurality of fuel cell stacks; and fuel gas circulation paths configured to return the fuel gas that has not been used in the respective plurality of fuel cell stacks to the respective fuel gas supply paths, wherein each of the plurality of fuel cell stacks includes at least one power generation output variable fuel cell stack whose power generation output is variably set to be operated using the fuel gas supplied through corresponding one of the fuel gas supply paths, and at least one power generation output constant fuel cell stack that is operated at a constant power generation output using the fuel gas supplied through corresponding one of the fuel gas supply paths, a flow rate regulator configured to adjust a flow rate of the fuel gas is provided on corresponding one of the fuel gas circulation paths configured to return the fuel gas that has not been used to be exhausted from the at least one power generation output variable fuel cell stack to the corresponding one of the fuel gas supply paths of the at least one power generation output variable fuel cell stack, and an ejector is provided on a merging portion of corresponding one of the fuel gas supply paths of the at least one power generation output constant fuel cell stack and corresponding one of the fuel gas circulation paths configured to return the fuel gas that has not been used to be exhausted from the at least one power generation output constant fuel cell stack to the corresponding one of the fuel gas supply paths of the at least one power generation output constant fuel cell stack. a plurality of fuel cell stacks; fuel gas supply paths configured to supply fuel gas to the respective plurality of fuel cell stacks; and fuel gas circulation paths configured to return the fuel gas that has not been used in the respective plurality of fuel cell stacks to the respective fuel gas supply paths, wherein each of the plurality of fuel cell stacks includes at least one power generation output variable fuel cell stack whose power generation output is variably set to be operated using the fuel gas supplied through corresponding one of the fuel gas supply paths, and at least one power generation output constant fuel cell stack that is operated at a constant power generation output using the fuel gas supplied through corresponding one of the fuel gas supply paths, a flow rate regulator configured to adjust a flow rate of the fuel gas is provided on corresponding one of the fuel gas circulation paths configured to return the fuel gas that has not been used to be exhausted from the at least one power generation output variable fuel cell stack to the corresponding one of the fuel gas supply paths of the at least one power generation output variable fuel cell stack, and an ejector is provided on a merging portion of corresponding one of the fuel gas supply paths of the at least one power generation output constant fuel cell stack and corresponding one of the fuel gas circulation paths configured to return the fuel gas that has not been used to be exhausted from the at least one power generation output constant fuel cell stack to the corresponding one of the fuel gas supply paths of the at least one power generation output constant fuel cell stack.