Abstract: In a method of manufacturing a semiconductor device, there are comprised the steps of forming an oxidation preventing layer on a surface of a semiconductor substrate, forming a first window in the oxidation preventing layer, placing the semiconductor substrate in a first atmosphere in which an oxygen gas and a first amount of a chlorine gas are flown through and then heating the semiconductor substrate at a first temperature such that a first selective oxide film is to grown by thermally oxidizing the surface of the semiconductor substrate exposed from the first window, forming a second window by patterning the oxidation preventing layer, and placing the semiconductor substrate in a second atmosphere in which the oxygen gas and a second amount, which is larger than the first amount, of the chlorine gas are flown through and then heating the semiconductor substrate at a second temperature such that a second selective oxide film is formed and that a thickness of the first selective oxide film formed below the f