A heat treatment apparatus is provided with a reaction tube having a furnace opening formed at a lower end thereof, a lid body configured to hermetically seal the furnace opening of the reaction tube, a heat treatment boat supported on the lid body through a leg, and a rotating shaft extending through the lid body. The rotating shaft is connected to a lower end of the leg and configured to rotate the leg. The lid body is provided with a surrounding ring protruding upward to surround the lower end of the leg. An inert gas is supplied from an inert gas supply unit to a space between the lid body and the rotating shaft and discharged from a space between the lower end of the leg and the surrounding ring into the reaction tube.