Fuel cell system and method of controlling fuel cell system

A method of controlling a fuel cell system comprising:

• acquiring a temperature related to a fuel cell having a fuel gas flow path inside;
• when the acquired temperature is below a specified temperature, increasing fuel gas concentration in the fuel gas flow path toward a first target concentration by activating a fuel-gas-concentration increasing mechanism configured to increase fuel gas concentration in the fuel gas flow path by using electric power of a secondary battery;
• when the fuel gas concentration reaches equal to or more than a second target concentration lower than the first target concentration, increasing the fuel gas concentration until the fuel gas concentration comes to the first target concentration or more by starting power generation by the fuel cell and activating the fuel-gas-concentration increasing mechanism with electric power from the fuel cell;
• when the fuel gas concentration reaches the first target concentration or more, controlling operation of the fuel cell in response to a power request;
• and
• when the acquired temperature is equal to or more than the specified temperature, controlling operation of the fuel cell in response to a power request.