An electrolyzer for high temperature electrolysis capable of operating in an allothermal mode including an enclosure, at least one electrolysis plate (8) including an anode and a cathode in combination and means for heating an active fluid intended to undergo a high temperature electrolysis, characterized in that the enclosure is capable of maintaining an electrolyte bath under high or very high pressure of several tens of bars, in that said heating means ( 10) are positioned in the enclosure and use a heat transfer fluid.