NOVELTY - The hydrogen producing apparatus (12) has a first cell device (24a) having first unit cells (22a) stacked in gravitational direction, and a second cell device (24b) having second unit cells (22b) also stacked in gravitational direction. The second cell device is coupled to the lower end of the first cell device in gravitational direction. The first cell device serves as a water electrolysis cell to electrolyze water. The second cell device serves as a water permeation cell to cause moisture on the cathode side to permeate toward an anode side.    USE - High-pressure hydrogen producing apparatus for generating hydrogen gas by water electrolysis. Can be used for electrolyzing water supplied to an anode side to generate oxygen on the anode side and hydrogen with a pressure higher than the pressure of oxygen on a cathode side.    ADVANTAGE - Eliminates the need for large equipment for discharging the high-pressure water, since the high-pressure hydrogen producing apparatus forms a differential pressure type water electrolysis apparatus so that high-pressure water can be permitted to permeate through the electrolyte membrane or electrode assembly to the lower-pressure and can be discharged efficiently. Ensures reduced size and simplified structure of the whole high-pressure hydrogen producing apparatus due to effectively reduced number of components of the high-pressure hydrogen producing apparatus. Includes a second cell unit that can be easily coupled to the first cell unit because the second cell unit is structured in a way similar to the fabrication of the first cell unit.    DETAILED DESCRIPTION - The first cell device includes an electrolyte membrane, an anode electrode catalyst layer and an anode current collector provided on a first surface of the electrolyte membrane, and a cathode electrode catalyst layer and a cathode current collector provided on a second surface of the electrolyte membrane. The second surface is provided on an opposite side of the electrolyte membrane to the first surface in the first cell device. The second cell device includes an electrolyte membrane, an anode current collector provided on a first surface of the electrolyte membrane of the second cell device, and a cathode current collector provided on a second surface of the electrolyte membrane of the second cell device. The second surface is provided on an opposite side of the electrolyte membrane to the first surface in the second cell device.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic illustration showing the configuration of a water electrolysis system including the high-pressure hydrogen producing apparatus.    High-pressure hydrogen producing apparatus (12)    First unit cells (22a)    Second unit cells (22b)    First cell device (24a)    Second cell device (24b)    Insulating plates (28a,28b)