A water electrolysis apparatus comprising: an electrolytic bath configured to electrolyze water, using an electrolytic membrane and generate oxygen and hydrogen on one surface side and the other surface side of the electrolytic membrane, respectively; an oxygen gas-liquid separation tank configured to separate oxygen generated on the one surface side from water; a conduit for oxygen disposed to the oxygen gas-liquid separation tank and including an oxygen pressure control valve; a first opening degree adjustment unit configured to control an opening degree of the oxygen pressure control valve; a hydrogen gas-liquid separation tank configured to separate hydrogen generated on the other surface side from water; a conduit for hydrogen disposed to the hydrogen gas-liquid separation tank and including a hydrogen pressure control valve; a second opening degree adjustment unit configured to control an opening degree of the hydrogen pressure control valve; a first tank internal pressure detector configured to detect tank internal pressure in the oxygen gas-liquid separation tank; a second tank internal pressure detector configured to detect tank internal pressure in the hydrogen gas-liquid separation tank; characterised in that the water electrolysis apparatus comprises: a current detector configured to detect current supplied to the electrolytic bath for the electrolysis, wherein the apparatus further comprises a) a first feedback control configured to control the opening degree of the oxygen pressure control valve, based on tank internal pressure detected by the first tank internal pressure detector in such a way that a difference between the tank internal pressure and a predetermined target pressure decreases, and b) a first feedforward control configured to control the opening degree of the oxygen pressure control valve, based on current detected by the current detector, and c) a second feedback control configured to control the opening degree of the hydrogen pressure control valve, based on tank internal pressure detected by the second tank internal pressure detector in such a way that a difference between the tank internal pressure and a predetermined target pressure decreases, and d) a second feedforward control configured to control the opening degree of the hydrogen pressure control valve, based on current detected by the current detector. an electrolytic bath configured to electrolyze water, using an electrolytic membrane and generate oxygen and hydrogen on one surface side and the other surface side of the electrolytic membrane, respectively; an oxygen gas-liquid separation tank configured to separate oxygen generated on the one surface side from water; a conduit for oxygen disposed to the oxygen gas-liquid separation tank and including an oxygen pressure control valve; a first opening degree adjustment unit configured to control an opening degree of the oxygen pressure control valve; a hydrogen gas-liquid separation tank configured to separate hydrogen generated on the other surface side from water; a conduit for hydrogen disposed to the hydrogen gas-liquid separation tank and including a hydrogen pressure control valve; a second opening degree adjustment unit configured to control an opening degree of the hydrogen pressure control valve; a first tank internal pressure detector configured to detect tank internal pressure in the oxygen gas-liquid separation tank; a second tank internal pressure detector configured to detect tank internal pressure in the hydrogen gas-liquid separation tank; characterised in that the water electrolysis apparatus comprises: a current detector configured to detect current supplied to the electrolytic bath for the electrolysis, wherein the apparatus further comprises a) a first feedback control configured to control the opening degree of the oxygen pressure control valve, based on tank internal pressure detected by the first tank internal pressure detector in such a way that a difference between the tank internal pressure and a predetermined target pressure decreases, and b) a first feedforward control configured to control the opening degree of the oxygen pressure control valve, based on current detected by the current detector, and c) a second feedback control configured to control the opening degree of the hydrogen pressure control valve, based on tank internal pressure detected by the second tank internal pressure detector in such a way that a difference between the tank internal pressure and a predetermined target pressure decreases, and a current detector configured to detect current supplied to the electrolytic bath for the electrolysis, wherein the apparatus further comprises a) a first feedback control configured to control the opening degree of the oxygen pressure control valve, based on tank internal pressure detected by the first tank internal pressure detector in such a way that a difference between the tank internal pressure and a predetermined target pressure decreases, and b) a first feedforward control configured to control the opening degree of the oxygen pressure control valve, based on current detected by the current detector, and c) a second feedback control configured to control the opening degree of the hydrogen pressure control valve, based on tank internal pressure detected by the second tank internal pressure detector in such a way that a difference between the tank internal pressure and a predetermined target pressure decreases, and