Abstract: In a furnace tube of a heat treatment apparatus, a mixture gas containing 5 to 25 vol %, preferably 20 vol % of O.sub.2 and 95 to 75 vol %, preferably 80 vol % of N.sub.2 is supplied to maintain the interior space of the heat treatment furnace tube in atmosphere of the mixture gas. Then, a semiconductor wafer is inserted in the furnace tube. Subsequently, the interior space of the furnace tube is purged by an N.sub.2 gas, and thereafter, vacuum is introduced. At this condition, a polycrystalline silicon layer is formed by way of low pressure chemical vapor deposition method. Thereafter, the interior space is purged by the N.sub.2 gas and the semiconductor wafer is removed from the furnace tube. By this, penetration of the ambient air into the furnace tube upon inserting the semiconductor wafer therein can be prevented to permit formation of uniform thickness of natural oxide layer.