A water electrolysis system includes a water electrolysis stack that electrolyzes water and generates hydrogen, a dehumidification device that adsorbs and removes moisture contained in the hydrogen generated by the water electrolysis stack, a wet line that supplies the hydrogen generated by the water electrolysis stack to the dehumidification device, a compression device that gradually increases pressure of the hydrogen whose moisture is removed in the dehumidification device, and a purge line that takes out a part of the hydrogen whose pressure is being increased, in the compression device, allows it to pass through the dehumidification device, and allows it to join the wet line.