To form a sufficiently round shape at the upper edge of a trench and to suppress level difference in the vicinity of the trench when it is filled with an insulating material in the process for fabricating a semiconductor device forming an isolation region of STI structure.

The process for fabricating a semiconductor device comprises a step for forming a trench 18 by etching a silicon substrate 11 using a sidewall insulating film 17 consisting of a laminate of at least a silicon oxide film 15 and a silicon nitride film 16 as a mask, a step for fomring a ringlike recess 19 in the sidewall of the trench 18 by etching the silicon oxide film 15 exposed from the silicon nitride film 16 in the trench 18, a step for forming a thermal oxidation film 20 by oxidizing the surface of the silicon substrate 11 exposed in the trench 18 including the recess 19, and a step for depositing an HDP-CVD film 21 on the entire surface including the trench 18.