PURPOSE: To prevent a semiconductor wafer from becoming defective when the outside air is mixed by a method wherein many gas curtain holes facing a part near the axis center are made on an inner periphery of a furnace inside tube near an opening part and the gas curtain holes are arranged in such a way that a mixing region of an inert gas exists at the inside of the opening part.

CONSTITUTION: Many gas curtain holes 20 facing 8 part near the axis center are formed on an inner periphery at a sufficiently inner part from an opening part where a furnace lid 13 is installed; they are connected by a duct 20a at their outer periphery; an inert gas is introduced. The gas curtain holes 20 on the side closest to the opening part are situated in such a way that a mixing region exists at the inside of the opening part. The ring-shaped gas curtain boles 20 spout a gas toward nearly one point on the axis center; accordingly, a gas stream from the individual gas curtain holes 20 interferes and is mixed as it approaches closer to the axis center; it involves a gas in the front and the rear and forms a mixing region; it forms a slope that two thin cones are piled up; it is pushed by a carrier gas from a tail tube and by an increase in its own volume; as it is shifted to the side of the opening part, the cones become thick; it does not involve the outside air.