PURPOSE: To flatten the surface of a substrate by forming the first nitriding film and the second nitriding film, which is larger in its etching speed than the first film, on a semiconductor substrate and forming a selective oxidizing film deeply twice in the substrate.

CONSTITUTION: The first selective oxidizing film 4 is formed deeply inside a substrate excluding an element formation region, and the whole selective oxidizing film 4 is etched/away and a CVD nitriding film 5 as the second nitriding film is piled on the whole surface including end parts of the substrate 1, the CVD nitriding film 5 on the inner surface of the substrate 1 and on the element formation region of it is removed by just etching. Successively, 8 remaining heat nitriding film 3 and a CVD nitriding film 5 remaining on the side part are used as masks for selective oxidation, and the second thick selective oxidizing film 8 is formed again inside the substrate excluding the element formation region, and this selective oxidizing film 8 over the vicinity of the surface of the substrate is oxidized. Hence, inequality and step in the surface of the semiconductor substrate can be flattened.