To solve a problem that the reliability of a gate oxide film is lowered in comparison with an oxide film formed by one time of gate oxidization when the gate oxide film is formed by plural times of gate oxidizing processes, in the case of forming the gate oxide films of different thickness on the same wafer.

An isolated oxide film 2 is formed on a wafer, next, all the surface of the wafer 1 is oxidized, and a first gate oxide film 5 is formed in first and second transistor regions 3 and 4. Next, only the first transistor region 3 is covered with photoresist 6, the surface of the first gate oxide film 5 existent in the second transistor region 4 is etched, and a thin second gate oxide film 7 is formed. Next, by removing the photoresist 6, the thick first gate oxide film 5 can be formed in the first transistor region 3, and the thin second gate oxide film 7 can be formed in the second transistor region. In this case, both the first gate oxide film 5 and the second gate oxide film 7 are formed by one time of oxidization, and the highly reliable formation of the gate oxide films is enabled.