A water electrolysis system includes a water electrolysis apparatus, a low-pressure gas-liquid separator, a high-pressure gas-liquid separator, water pipe, and a decompression water supply device. The high-pressure gas-liquid separator separates the hydrogen received from a cathode of the water electrolysis apparatus and permeation water that has permeated from the anode. The water pipe connects the high-pressure gas-liquid separator and the low-pressure gas-liquid separator and is used for returning the water from the high-pressure gas-liquid separator to the low-pressure gas-liquid separator. The decompression water supply device is arranged at the water pipe and is to decompress the water discharged from the high-pressure gas-liquid separator.