A differential pressure type high pressure water electrolysis apparatus includes a seal member, which is sandwiched between a cathode side separator and a membrane electrode assembly, and surrounds a cathode electrode catalyst layer, and a pressure resistant member surrounding the seal member from an outer side thereof. A surface pressure applying member is interposed between the seal member and the pressure resistant member. The surface pressure applying member receives a pressing force from the seal member, and applies pressure to the membrane electrode assembly.