An electrolyser for high-temperature electrolysis configured to operate in an allothermal mode, including an enclosure configured to maintain an electrolytic bath under high or very high pressure of several tens of bars, in which at least one electrolysis plate is arranged, and a heater internal to the enclosure. The electrolysis plate includes a plurality of electrolysis cells lying side by side in substantially one same plane, each electrolysis cell including an anode and a cathode. The heater uses a heat-carrier fluid.