NOVELTY - The apparatus (10) has flat packing (66) set between solid polymer electrolyte membrane (40) and outer circumference edge portion of anode side separator (34). One of the outer circumference edge portions of anode side separator and water channel (56) has smaller dimension at depth from bottom of anode chamber (52) than thickness of anode side element (60). An O-ring (68) has same thickness of anode side element during operation of high-pressure water electrolysis apparatus. The O-ring is set in anode chamber to shield between anode chamber and high-pressure hydrogen communication hole (38c).    USE - High-pressure water electrolysis apparatus for electrically decomposing water and generating oxygen and high-pressure hydrogen.    ADVANTAGE - The desired seal function is secured with simple and economical configuration since processing precision of the anode side separator does not have to be set high. Suppresses damage of the O-ring since large numbers of components are not in contact with the O-ring. Improves assembling workability of anode side element including multiple components since recessed portion is formed in the water channel and anode current collector and protective sheet are housed and disposed in recessed portion. Improves processability of anode side element since anode side element makes up the flat plane member as a whole and O-ring has the same thickness as with the anode side element. The smoothness of the surface of anode side element is maintained in sure manner since anode current collector held in the frame portion is kept from being crushed in the event that anode current collector is pressed at the time of electrolysis operation. The O-ring is effectively prevented from withdrawing under working of the disc spring holder at the time of depressurization of the cathode chamber since O-ring is in contact with the disc spring holder.    DESCRIPTION OF DRAWING(S) - The drawing shows an exploded perspective view of the unit cell making up the high-pressure water electrolysis apparatus.    High-pressure water electrolysis apparatus (10)    Anode side separator (34)    High-pressure hydrogen communication hole (38c)    Solid polymer electrolyte membrane (40)    Anode chamber (52)    Water channel (56)    Anode side element (60)    Flat packing (66)    O-ring (68)