A water electrolysis system includes a water electrolysis apparatus for electrically decomposing water to generate oxygen and high-pressure hydrogen having a pressure higher than the oxygen, a gas-liquid separator connected to a hydrogen pipe which discharges the high-pressure hydrogen from the water electrolysis apparatus, for separating water contained in the high-pressure hydrogen, a high-pressure hydrogen outlet pipe for delivering the high-pressure hydrogen separated from water from the gas-liquid separator, a water drainage line for discharging the water from the gas-liquid separator, and a gas depressurizing line connected to the gas-liquid separator, for degassing the gas-liquid separator before the water is discharged from the water drainage line into the water drainage line.