NOVELTY - The system (10) has a high-pressure hydrogen production unit (12) for electrolyzing water and a gas-liquid separation unit (22) for removing water contained in the high-pressure hydrogen, placed on a hydrogen pipe (24) for discharging the high-pressure hydrogen from the high-pressure hydrogen production unit. A hydrogen supply pipe (24) transfers dewatered high-pressure hydrogen from the gas-liquid separation unit. A cooling unit (26) for variably controlling temperature of the high-pressure hydrogen to adjust the humidity of the high-pressure hydrogen, is placed on the hydrogen supply pipe.    USE - Water electrolysis system used to produce hydrogen used as e.g. a fuel gas for performing a power generation reaction.    ADVANTAGE - Provides a water electrolysis system that minimizes energy consumption for hydrogen dehumidification, exhibits improved economic efficiency, convenience and operational efficiency and that is capable of easily lowering the water amount in the unit to a threshold value or less even in the event that water is introduced into a hydrogen storage unit. Hydrogen is not cooled excessively by the cooling unit even when the pressure of the high-pressure hydrogen increases or decreases due to any of various factors since the high-pressure hydrogen can be cooled sufficiently in the high-pressure hydrogen supply pipe depending on detected pressure. Power consumption of the Peltier dehumidifier for dehumidifying hydrogen, and the amount of cooling water supplied to the heat exchanger is reduced to curb energy consumption, and thus, economic efficiency and convenience are improved.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for operating a water electrolysis system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic structural view of a water electrolysis system.    Water electrolysis system (10)    High-pressure hydrogen production unit (12)    Gas-liquid separation unit (22)    Hydrogen pipe (24)    Hydrogen supply pipe (24)    Cooling unit (26)