NOVELTY - The system (10) has a high-pressure hydrogen processing apparatus including a gas-liquid separator (18) and moisture-adsorption apparatus (22), connected to a hydrogen outlet of a water electrolysis apparatus (14). An inlet valve i.e. check valve (40), and an outlet valve i.e. back-pressure valve (42), are connected respectively to inlet and outlet ports of the processing apparatus. A pressure releasing device (52) connected between a hydrogen outlet and the inlet valve releases pressure of the hydrogen from the electrolysis apparatus independently from the processing apparatus.    USE - Water electrolysis system for electrolyzing water to generate a hydrogen gas for use as a fuel gas for a solid polymer electrolyte fuel cell of a vehicle i.e. car (from drawings).    ADVANTAGE - The pressure releasing device connected between the hydrogen outlet and the inlet valve releases pressure of the hydrogen from the electrolysis apparatus independently from the processing apparatus, thus eliminating depressurization of the separator and the moisture-adsorption apparatus containing a relatively large amount of high-pressure hydrogen, and minimizing amount of high-pressure hydrogen wasted, and hence ensuring water electrolysis in an efficient and economical manner. The pressure releasing device ensures that the high-pressure hydrogen from the water electrolysis apparatus is not depressurized abruptly, thus preventing the solid polymer electrolyte membranes and seals of the water electrolysis cells from being damaged by an abrupt expansion of hydrogen.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for shutting down a water electrolysis system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a water electrolysis system to which a system shutdown method is applied.    Water electrolysis system (10)    Water electrolysis apparatus (14)    Gas-liquid separator (18)    Moisture-adsorption apparatus (22)    Check valve (40)    Back-pressure valve (42)    Pressure releasing device (52)