By covering ends of a field insulating film in a region where a MOS transistor having a relatively thin gate insulating film is formed with a relatively thick gate insulating film, a channel region of the MOS transistor having the relatively thin gate insulating film is set apart from an inversion-preventing diffusion layer formed under the field insulating film so as not to be influenced by film thickness fluctuation of the field insulating film, etching fluctuation of the relatively thick gate insulating film, and impurity concentration fluctuation at both sides of the channel due to the inversion-preventing diffusion layer.