Abstract: An under-heater type furnace for maintaining a molten metal at a high temperature, comprising a refractory vessel made of an integral castable material for containing the molten metal, a heat insulating layer made of a ceramic fiber material and fixed outside of the refractory vessel for covering thermally the refractory vessel, a heater, a protective tube made of ceramics and having a closed end and an open end for housing the heater and protecting the heater from the molten metal in the refractory vessel, the refractory vessel having a first support at a lower side portion thereof and a second support at an opposite side portion thereof, the first support having a tapered opening wall which is outwardly widened, a sealing material disposed between the tapered opening wall and the protective tube for fixing the protective tube relative to the vessel, the second support supporting the closed end of the protective tube in such a manner that the closed end of the protective tube can freely move relative to the