NOVELTY - A water electrolysis system (10) comprises a high-pressure water electrolysis apparatus (12) including an electrolyte membrane (48) and power feeders sandwiching the electrolyte membrane in between; a water circulation apparatus (14) to circulate the water to the high-pressure water electrolysis apparatus; and a vapor-liquid separation apparatus (16) comprising a reservoir (78), a blower (86), and a movable wall (90) disposed in the reservoir.    USE - A water electrolysis system.    DETAILED DESCRIPTION - A water electrolysis system (10) comprises:    (A) a high-pressure water electrolysis apparatus (12) to generate oxygen on an anode side and generate hydrogen on a cathode side by electrolyzing water, the high-pressure water electrolysis apparatus including an electrolyte membrane (48) and power feeders sandwiching the electrolyte membrane in between, the hydrogen having a pressure higher than a pressure of the oxygen;    (B) a water circulation apparatus (14) to circulate the water to the high-pressure water electrolysis apparatus; and    (C) a vapor-liquid separation apparatus (16) to separate a gas component discharged from the anode side of the high-pressure water electrolysis apparatus from the water in the water circulation apparatus, the vapor-liquid separation apparatus comprising a reservoir (78) having an inlet port in its lower part through which the gas component from the high-pressure water electrolysis apparatus and the water are introduced; a blower (86) to supply dilution air to the reservoir from an upper part of the reservoir; and a movable wall (90) disposed in the reservoir, and vertically movable in accordance with a water level in the reservoir and allowing the gas component to pass.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic view of the water electrolysis system.    Water electrolysis system (10)    High-pressure water electrolysis apparatus (12)    Water circulation apparatus (14)    Vapor-liquid separation apparatus (16)    Electrolyte membrane (48)    Reservoir (78)    Movable wall (90)