PURPOSE:To prevent the contamination of a furnance due to the reflux of air from a chamber and accordingly improve the electric characteristic of an oxidized wafer, by providing an external lid, having an aperture part smaller than the inner diameter of a reaction tube, at the out let end part of the reaction tube. CONSTITUTION:Using an oxidation furnace kept at 1,000 deg.C for 100hr or more, an Si wafer is oxidized (heat treatmentI) resulting in the measurement of life time. Next, after the temperature of the furnace is decreased down to 700 deg.C and kept for 13hr (heat treatment II), it is set again to 1,000 deg.C, and the Si wafer is oxidized (heat treatment III) resulting in the measurement of life time. The curve A represents the result of measurement, when using a cap (external lid) 1 in the heat treatment II; the curve B the result of measurmnt, when using a cap (internal lid) 2. When using the internal lid 2, the life time value is lower than when using the external lid 1, and therefore it is provied that the contamination becomes in a large amount. It can be considered that, when using the internal lid, ths reason is that the air in the chamber enters in reflux from the clearance between the lid and the quartz tube 3 resulting in the dispersion and scattering of the small amount impurity contained in the air.