NOVELTY - The reservoir (20) has a drain valve (30) with an inlet and outlet ports that are formed on a rear end and front side walls of the valve housing, respectively. A wind receiving plate is installed on a front surface of the valve housing and is moved by traveling wind during traveling of vehicle. A shielding plate is integrally movable with the wind receiving plate to open and close the inlet port. A coil spring is mounted between the rear surface of wind receiving plate and rear end wall of the valve housing.    USE - Cooling water reservoir for fuel cell vehicle (claimed). Can also be used in motor vehicles such as passenger automobiles e.g. sports utility vehicles (SUV), buses, trucks, commercial vehicles, water craft such as boats and ships, aircraft, electric vehicle, hydrogen-powered vehicles, hybrid vehicles such as petrol-powered vehicles, electric-powered vehicles, plug-in hybrid electric vehicles, etc.    ADVANTAGE - The water generated in the fuel cell stack during operation of the fuel cell system and flowing in the reservoir can be discharged through the drain valve automatically in real-time during traveling of vehicle, so that the water in the reservoir housing is effectively prevented from overflowing due to the inflow of the product water generated in the fuel cell stack during operation of the fuel cell system. Since the drain valve is automatically opened by the traveling wind while the vehicle is running, the need for installing expensive electronic valve is eliminated and hence a simple configuration is realized. The unwanted water from the water reservoir can be discharged automatically by inexpensive mechanical drain valve, without any controller or power supply in the vehicle traveling condition, so that the manufacturing cost is reduced and a lightweight structure is realized.    DESCRIPTION OF DRAWING(S) - The drawing shows an explanatory view of the drain valve installed in the reservoir housing.    Reservoir (20)    Reservoir housing (21)    Drain valve (30)