Provided are a water electrolysis method and a water electrolysis device in which mixing of the generated hydrogen and oxygen is greatly reduced and which have a high electrolysis efficiency, while being simplified in structure. In the water electrolysis method and water electrolysis device, water is electrolyzed by supplying water to the cathode side of an electrolytic membrane including a solid polymer membrane provided with a catalyst layer on a surface thereof and creating a potential difference between both surfaces of the electrolytic membrane. The temperature-controlled water is supplied only to the cathode side of the electrolytic membrane, while controlling the difference in pressure between both surfaces of the electrolytic membrane to 50 kPa or less.