PURPOSE: To accurately form an offset gate structure by providing an insulating film having a prescribed shape on a semiconductor substrate and deciding the difference between a source- and drain-side recessing lengths of the substrate based on the side etching amount of the insulating film.

CONSTITUTION: An insulating film 3 having a prescribed shape is formed on an epitaxial semiconductor substrate 1 and the difference between the source- and drain-side recessing length of the substrate 1 is decided based on the side etching amount of the film 3. For example, a silicon oxide film 2 and silicon nitride film 3 are grown on a GaAs substrate 1 and, after a desired pattern 4 is formed by patterning a photoresist film formed on the film 3, the film 3 is dry-etched. Then, after growing another oxide film 5 and coating the film 5 with another photoresist film 6, a gate opening 7 is formed and the oxide films 2 and 5 and nitride film 3 are dry-etched. Thereafter, a recessed area 9 is formed by etching off the substrate by a required amount after side etching the film 3 by a required amount and removing the oxide film 2 from an opening 8.