A fuel cell system has a fuel cell, an air-refrigerant heat exchanger, and primary and secondary expansion valves. The air-refrigerant heat exchanger performs heat exchange between exhaust air discharged from the fuel cell and a refrigerant in a refrigeration cycle. The primary expansion valve reduces the pressure of the refrigerant at an upstream side of the air-refrigerant heat exchanger. The secondary expansion valve reduces the pressure of the refrigerant at a downstream side of the air-refrigerant heat exchanger. While the air-refrigerant heat exchanger performs the heat-exchange from the air to the refrigerant, the primary expansion valve controls a reduced magnitude of the pressure of the refrigerant so that a temperature of the refrigerant is set to a specified value within a range of 0.degree. C. to 5.degree. C., preferably set to 0.degree. C., at which water component contained in the air does not freeze.