a method for forming an isolation region on a semiconductor substrate which comprises steps of forming first a groove in said substrate , filling the groove with a first insulating film in a manner which may inevitably leave a gap between the wall of the groove and the each side of the first insulating film , forming a second insulating film on the surface of said substrate including said groove , and removing a surface layer of said second insulating film thereby forming an isolation region in the groove filled completely with the first and second insulating films .