A fuel cell system includes a fuel cell stack constituted by cells, each of the cells includes a fuel electrode, an air electrode, and an electrolyte, and generate electric power through a reaction of a fuel gas and air, a casing that houses the fuel cell stack, a temperature detector that detects a first temperature, the first temperature is a temperature of the fuel cell stack or inside the casing, and a controller. The controller controls based on the first temperature so as to allow an operation at a first predetermined temperature. The controller controls such that the first temperature reaches a temperature higher than or equal to a second predetermined temperature for a predetermined time. The second predetermined temperature is a temperature at which 475° C. embrittlement that occurs on stainless steel is eliminated. The first predetermined temperature is lower than the second predetermined temperature.