A semiconductor device manufacturing method includes: forming a first active region and a second active region in a semiconductor substrate; forming a first gate insulating film on the first active region and a second gate insulating film thinner than the first gate insulating film on the second active region by using material containing silicon oxide; forming first and second gate electrodes on the first and second gate insulating films respectively; forming an insulating film on the semiconductor substrate, and anisotropically etching the insulating film to leave first side wall insulating films on side walls of the first and second gate electrodes; removing the first side wall insulating film on the first gate electrode; and after removing the first side wall insulating film on the first gate electrode, thermally treating in an oxidizing atmosphere the semiconductor substrate to form a second side wall insulating film on the first gate electrode.