A fuel cell system according to the present invention comprises: a fuel cell device; a heat medium circulation path (9) through which a heat medium that performs a heat exchange with an off gas is circulated; a first heat exchanger that is provided in the heat medium circulation path (9) and produces condensed water; a cooling mechanism that is provided in the heat medium circulation path (9) and cools the heat medium before being supplied to the first heat exchanger; a first temperature detector (28) that detects the temperature of the heat medium after being cooled by the cooling mechanism and before being supplied to the first heat exchanger; and a controller (101) that controls the cooling mechanism so that the temperature detected by the first temperature detector (28) becomes a first temperature set for producing condensed water necessary for a modification reaction in the first heat exchanger.
