PURPOSE: To improve TFT characteristics, by locally increasing the thickness of a gate insulating film to relieve both a drain electric field and a gate electric field.

CONSTITUTION: In a thin film transistor 10, a gate insulating film 12 and a gate electrode 13 are laminated, and source.drain regions 14, 15 are formed on the semiconductor substrate on both sides of the gate electrode 13. A gate insulating film 12a under both sides of the gate electrode 13 is formed thicker to the semiconductor substrate 11 side than a gate insulating film 12b op the central side under the gate electrode 13. In order to form partially thick the gate insulating film 12, a thermal oxidation process is used, or heat treatment is performed after oxidizing impurities are obliquely ion-implanted in the semiconductor substrate 11 under both sides of the gate electrode 13. In other case, by obliquely irradiating the semiconductor substrate 11 in an oxidizing atmosphere with an energy beam, the gate insulating film 12a under both sides of the gate electrode 13 is formed thicker than the gate insulating film 12b on the central side under the gate electrode 13.