A fuel cell system (100) includes: a first circulation passage (2) through which circulates a primary cooling medium to recovery the exhaust heat of the fuel cell system (100); a heat storage unit (5) configured to store a secondary cooling medium which has recovered heat from the primary cooling medium; a heat exchanger (4) where heat exchange takes place between the primary cooling medium and the secondary cooling medium; a second circulation passage (30A), not passing through the heat storage unit (5), through which circulates the secondary cooling medium to exchange heat with the primary cooling medium; a first circulator (6) configured to cause the primary cooling medium to circulate through the first circulation passage (2); a second circulator (8) configured to cause the secondary cooling medium to circulate through the second circulation passage (30A); a heater (200) configured to heat the secondary cooling medium; and a controller (40) configured to perform, for inhibiting freezing of the primary cooling medium, a first process so that the heater (200) is operated and the first and the second circulators (6) and (8) are operated.