PURPOSE:To reduce microroughness and prevent the change of structure and electric characteristics of an element formed in a semiconductor device. CONSTITUTION:In the manufacturing method of a semiconductor device having a pre-oxidation process wherein impurities on the silicon wafer surface are eliminated by eliminating an oxide film after the oxide film is formed on a silicon wafer, the oxide film in the pre-oxidation process is formed in an oxidizing atmosphere containing O2 gas, H2 gas and GeH4 gas. Since the GeH4 gas is contained in the oxidizing atmosphere, the softening point of the silicon wafer can be decreased, Thereby microroughness on the silicon wafer surface can be reduced. Since the pre-oxidation process can be performed at a low temperature in a short time, structure and electric characteristics of an element formed in a semiconductor device are not changed.