NOVELTY - The apparatus has a water tank (27) connected to a cooling unit (20) of the polymer-electrolyte fuel cell (17) such that pure water (28) is returned to the water tank from the cooling unit. A thermometer (38) detects the temperature in the water tank. A controller (40) judges whether the temperature detected value results in freezing and open-closes the shut-off valves (36,37) provided in the drain line (35) of the water tank based on a command generated according to the judgment result.    USE - For polymer-electrolyte fuel cell (PEFC) electric power generating apparatus used for motor vehicle, small-scale industry and household use.    ADVANTAGE - The freeze prevention of the circulating cooling water and the usage amount of heater at the time of apparatus stop are decreased. The heating-up time of the line of cooling water at the time of starting is shortened and the piping obstruction by freezing, crack etc., is prevented. The construction components of the heater for freeze prevention can be reduced and the power consumption in the time of countermeasure against freeze prevention can be reduced.    DETAILED DESCRIPTION - The apparatus has a humidifier (21) installed in the anode (19) of the fuel cell and entrance side of cathode (18). A pure water pump (29) pressurizes the cooling water in the water tank in an electric power generation unit (25). The controller opens the shut-off valves when the temperature of water tank or the temperature of the electric power generation unit results in freezing, and closes the shut-off valves when the temperature of water tank or the temperature of electric power generation unit turns into the temperature value without a fear of freezing. A heat exchanger (45) is provided in a storage tank (43) installed in the outer side of the electric power generation unit. The warm water circulation line (48) is connected to the heat exchanger. A heat exchanger (47) is provided inside the electric power generation unit. The non-return valves (52,53) are provided in the upstream of upstream heat exchanger in the circulation line of warm water and downstream of downstream heat exchanger to perform heat recess prevention of warm water. A heater (54) for freeze prevention is attached to feed water line and drain line.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram of the PEFC electric power generating apparatus.    Polymer-electrolyte fuel cell (17)    Cathode (18)    Anode (19)    Cooling unit (20)    Humidifier (21)    Electric power generation unit (25)    Water tank (27)    Pure water (28)    Pure water pump (29)    Drain line (35)    Shut-off valves (36,37)    Thermometer (38)    Controller (40)    Storage tank (43)    Heat exchangers (45,47)    Warm water circulation line (48)    Non-return valves (52,53)    Heater (54)