A water electrolysis system includes a water electrolysis device, a gas-liquid separator, a water-amount detector, a hydrogen storage device, a hydrogen storage device, a decompressing device, and a control device. The control device includes a residual capacity calculator, a hydrogen production amount calculator, and an electrolysis termination determiner. The residual capacity calculator is configured to calculate a residual capacity of the hydrogen storage device. The hydrogen production amount calculator is configured to calculate an amount of hydrogen produced by the water electrolysis device in a drainage period in which the amount of water in the gas-liquid separator increases to an upper limit from a lower limit. The electrolysis termination determiner is configured to terminate the water electrolysis process when the amount of hydrogen calculated by the hydrogen production amount calculator is greater than the residual capacity calculated by the residual capacity calculator.