Fuel cell system and related operating method

A fuel cell system and related operating method are disclosed wherein a water storage section (a water storage tank 33), that stores water to humidify a fuel cell 1, is provided with a heat transfer surface (52a to 55a, 52b to 54b) that is heated by hot medium supplied to a hot medium flow passage. The heat transfer surface is inclined such that a surface area surrounded by the heat transfer surface increases as awater level increases. Antifreeze solution forming coolant medium of the fuel cell is heated and used as hot medium of the hot medium f low passage.