A water electrolysis device of the present invention includes: an oxygen separation tank for storing demineralized water to be fed to the electrochemical cell; a demineralized water circulation line for feeding demineralized water from the oxygen separation tank to the electrochemical cell and returning demineralized water; which has been electrolyzed and remains in the electrochemical cell, to the oxygen separation tank; a demineralized water make-up line for feeding demineralized make-up water to one of the oxygen separation tank and the demineralized water circulation line; a first heat exchanger interposed in the demineralized water make-up line; and a first demineralized water discharge line having a proximal end communicated with one of the oxygen separation tank and the demineralized water circulation line, and a distal end extending through the first demineralized water discharge line to the outside, in which heat transfer between the demineralized make-up water fed through the demineralized water make-up line and demineralized water discharged to the outside through the first demineralized water discharge line is carried out at the first heat exchanger.