A fuel cell system having a fuel cell, coolant circulating means which circulates cooling water into the fuel cell, temperature sensors which detect a cooling water outlet temperature and a cooling water inlet temperature, and temperature adjusting means which adjusts a temperature of the fuel cell by controlling a flow rate of the coolant on the basis of the cooling water outlet temperature, is characterized by including abnormality determining means which determines whether or not abnormality occurs in the temperature sensor, and coolant temperature estimating means which, when the abnormality determining means determines that abnormality occurs in the temperature sensor, estimates the cooling water outlet temperature on the basis of physical information relating to operating states of the fuel cell.