NOVELTY - The system (10) has a water electrolysis device (12) utilized to electrolyze water to generate oxygen and high-pressure hydrogen with pressure higher than pressure of the oxygen. A hydrogen production amount calculator (86) of a control device (82) calculates an amount of hydrogen produced by the electrolysis device in a drainage period. An electrolysis termination determiner (88) of the control device terminates the water electrolysis process when the amount of hydrogen calculated by the calculator is greater than a residual capacity calculated by a residual capacity calculator (84).    USE - Water electrolysis system for directly supplying dry hydrogen to a fuel tank i.e. hydrogen storage device, in a fuel cell vehicle i.e. fuel cell electric automobile, through a nozzle.    ADVANTAGE - The system performs the drainage process when the amount of water in a gas-liquid separator has reached an upper limit, and the amount of hydrogen, which remains in the gas-liquid separator, is at minimum, so that the amount of hydrogen discharged from the gas-liquid separator is low, thus improving the system efficiency. The system controls the water electrolysis device in a simple manner, so that the hydrogen can be reliably and quickly supplied to an on-board hydrogen tank, thus supplying the hydrogen easily and economically to the fuel cell vehicle, and hence providing compact system and simple controlling operation.    DETAILED DESCRIPTION - The water electrolysis device is a differential-pressure-type water electrolysis device and a high-pressure hydrogen producing apparatus. An INDEPENDENT CLAIM is also included for a method for operating a water electrolysis system.    DESCRIPTION OF DRAWING(S) - The drawing shows a hydraulic circuit diagram of a water electrolysis system.    Water electrolysis system (10)    Water electrolysis device (12)    Control device (82)    Residual capacity calculator (84)    Hydrogen production amount calculator (86)    Electrolysis termination determiner (88)