A water electrolysis system includes: a water electrolysis tank; a cathode side gas passage through which a cathode side gas flows; a monitoring device configured to monitor at least one of a hydrogen concentration of the cathode side gas in the cathode side gas passage, an oxygen concentration of the cathode side gas in the cathode side gas passage, and an amount of electric power supplied to the water electrolysis tank; a hydrogen supply passage through which a hydrogen gas is supplied to the cathode side gas in the cathode side gas passage to increase the hydrogen concentration of the cathode side gas; and a flow regulating valve configured to regulate a flow rate of the hydrogen gas supplied through the hydrogen supply passage to the cathode side gas.