A water pump (4) recirculates water from a water tank (3) in a heat exchanger (2) and a fuel cell stack (1) of a fuel cell power plant via a recirculation passage (5) A water temperature sensor (13) detects a water temperature in the recirculation passage (5). When the water temperature is lower than a predetermined temperature, a controller (16) recirculates water to the water recirculation passage (5) by operating a water pump (4) in order to prevent freezing of water. It is preferred that a heater (15) is provided to heat the water recirculating in the recirculation passage (5).