In this manner, the excessive silicon dioxide film is completely removed off the substrate 1 and the thick insulating layer having the thickness of 10 ??m or more is left in the passive element forming region A1. [0122] The oxidation process shown in FIG. 12 is a process for forming an oxide film of 1 ??m or less which is simultaneously performed together with the LOCOS (i.e., localized oxidation