A method of manufacturing a semiconductor device has the steps of: forming a mushroom gate traversing an active region of a semiconductor substrate and having a fine gate and an expanded over gate formed thereon; coating a first organic material film on the semiconductor substrate; patterning the first organic material film and leaving the first organic material film only near the mushroom gate; coating a second organic (insulating) material film covering the left first organic material film; forming an opening through the second organic material film to expose the first organic material film; and dissolving and removing the first organic material film via the opening to form a hollow space in the second organic material film.