A water electrolysis system includes a water electrolysis apparatus including an electrolyte membrane. The electrolyte membrane is provided between an anode and a cathode. The water electrolysis apparatus is configured to generate oxygen on a side of the anode and hydrogen on a side of the cathode at a pressure higher than a pressure of the oxygen through electrolysis of water. A gas-liquid separation apparatus separates unreacted water and produced gas discharged from a water outlet of the water electrolysis apparatus. A water circulation apparatus circulates the water between the water electrolysis apparatus and the gas-liquid separation apparatus. The water circulation apparatus includes a return pipe having an on-off valve and connecting the water outlet and the gas-liquid separation apparatus. A hydrogen exhaust pipe is connected to the return pipe between the water outlet and the on-off value and extends upward from the water electrolysis apparatus.