Disclosed is a heat treatment apparatus which includes a processing vessel having a furnace throat at its bottom and adapted to accommodate process objects therein to perform a heat treatment to the process objects under reduced pressure, the processing vessel having a vessel main body made of quartz, a metallic lid adapted to support thereon a holder for holding a plurality of process objects so as to load and unload the holder into and from the processing vessel and to close and open the furnace throat, and an annular sealing member disposed on the lid to seal a gap between the lid and the furnace throat. A contact-preventing member is disposed between the lid and the furnace throat to prevent contact of the lid with the furnace throat due to squashing of the sealing member that would otherwise occur when an internal pressure of the processing vessel is reduced.