Disclosed is a hermetic container for thermal conversion reaction, including: a base plate in which utilities are installed; a vessel which, together with the base plate, forms a hermetic hot zone; a heater which is arranged in the hot zone; inlet and outlet holes through which reaction gas is supplied to and discharged from the hot zone; and a heat exchanger which is provided inside the vessel so that the reaction gas supplied to the hot zone via the inlet holes can absorb thermal energy transferred to the vessel to cool temperature of the vessel and at the same time be supplied to the hot zone as being heated. With this, when the reaction gas is supplied to the hot zone via the heat exchanger provided inside the vessel, the thermal energy transferred from the heater of the hot zone to the vessel and escaping outward is absorbed by the reaction gas supplied to the hot zone, thereby preventing the vessel from being heated at a limit temperature or over. Further, the reaction gas absorbing the thermal energy escaping to the outside of the vessel is heated and then supplied to the hot zone, thereby reducing power consumption of the heater.