A fuel cell system comprises a reformer for reforming a feed material to generate a reformed gas containing hydrogen, a material feed means for feeding the feed material to the reformer, a heater for heating the reformer, a fuel cell for generating a power by reacting the reformed gas supplied from the reformer with an oxidizing gas, and a temperature detector for detecting a temperature at a predetermined position in the fuel cell system, and in a first antifreezing operation mode, when the detector detects a temperature which is not higher than a threshold, the feed material heated in the reformer by the heater is caused to flow through flow passages of the reformed gas.