Fuel cell system and method for operating the same

A fuel cell system comprises a medium circulating passage (72) for supplying, in a circulating manner, a temperature adjusting medium to a fuel cell stack (12), and first and second temperature sensors (84a), (84b) for detecting medium temperatures at a temperature adjusting medium inlet (54a) and a temperature adjusting medium outlet (54b). The medium circulating passage (72) is provided with a pump (74) for circulating the medium, a heater (76) for heating the medium, a radiator (78) for cooling the medium, a bypass passage (80) for bypassing the radiator (78), and first through third opening/closing valves (88), (90), (92) for selectively supplying the temperature adjusting medium to the radiator (78) and the bypass passage (80).