FUEL CELL SYSTEM

To provide a fuel cell system configured to achieve both rapid cooling of a fuel cell at high temperatures and rapid heating of the fuel cell at the time of system start-up. In the fuel cell system, by controlling a three-way valve, a controller switches to any one of the following circulation systems: radiator circulation in which a refrigerant flows to a radiator through a first flow path, and third flow path circulation in which the refrigerant bypasses the radiator and flows to a second flow path through a third flow path; when the temperature of the refrigerant is equal to or less than a low temperature threshold, the controller switches from the radiator circulation to the third flow path circulation and closes a first valve; and when the temperature of the refrigerant becomes equal to or more than a high temperature threshold, the controller opens the first valve and circulate the refrigerant to flow through the reserve tank.