A high-pressure electrolyzer for generating hydrogen and oxygen is provided comprising a plurality of electrolysis units arranged in series, wherein each unit comprises a body of conductive metal made up of an assembly of interconnected horizontal and vertical tubes, said body constituting an electrode which is connectable to a source of DC electricity; wherein said assembly comprises three horizontal tubes and at least two vertical tubes, the vertical tubes each accommodating an elongated central electrode and a tubular membrane, wherein each vertical tube together with the central electrode, the membrane and an electrolyte constitute an electrolytic cell, the electrolytic cells within each unit being connected in parallel, wherein each unit further comprises at least two vertical tubes not accommodating central electrodes, the first vertical tube connecting the lower horizontal tube to the first upper horizontal tube and the second vertical tube connecting the lower horizontal tube with the second upper horizontal tube.