NOVELTY - The water electrolysis system comprises a water electrolysis apparatus (12) for electrically decomposing water to generate oxygen and high-pressure hydrogen having a pressure higher than the oxygen, a gas-liquid separator (52) connected to a hydrogen pipe (50), which discharges the high-pressure hydrogen from the water electrolysis apparatus for separating water contained in the high-pressure hydrogen, a high-pressure hydrogen outlet pipe (54) for delivering the high-pressure hydrogen separated from water by the gas-liquid separator, and a water drainage line (56) for discharging the water.    USE - Used as a water electrolysis system.    ADVANTAGE - The water electrolysis system is capable of effectively and economically reducing the hydrogen dissolved in discharged water, prevents high-pressure water discharged into the water drainage line, and increases the durability of the device.    DETAILED DESCRIPTION - The water electrolysis system comprises a water electrolysis apparatus (12) for electrically decomposing water to generate oxygen and high-pressure hydrogen having a pressure higher than the oxygen, a gas-liquid separator (52) connected to a hydrogen pipe (50), which discharges the high-pressure hydrogen from the water electrolysis apparatus for separating water contained in the high-pressure hydrogen, a high-pressure hydrogen outlet pipe (54) for delivering the high-pressure hydrogen separated from the water by the gas-liquid separator, a water drainage line (56) for discharging the water from the gas-liquid separator, and a gas depressurizing line connected to the gas-liquid separator for degassing the gas-liquid separator before the water is discharged from the gas-liquid separator into the water drainage line. The gas depressurizing line includes a first pressure reducing valve and a first on-off valve. The water drainage line includes a second pressure reducing valve and a second on-off valve. The water electrolysis system further comprises a water electrolysis apparatus depressurizing line branched from the hydrogen pipe upstream of the gas-liquid separator. An INDEPENDENT CLAIM is included for a method of operating a water electrolysis system.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a water electrolysis system (Drawing includes non-English language text).    Water electrolysis apparatus (12)    Hydrogen pipe (50)    Gas-liquid separator (52)    High-pressure hydrogen outlet pipe (54)    Water drainage line. (56)