This semiconductor device manufacturing method comprises the steps of: forming a thick gate oxide film (thick oxide film) in a first region of a substrate, forming a thin gate oxide film (thin oxide layer) in a second region, and then, applying oxynitridation to these gate oxide films; forming gate electrodes to 1d on these gate oxide films; and implanting an ion that contains nitrogen or nitrogen atoms into at least one part of an interface between the hick gate oxide film (thick oxide film) and the substrate before or after the step of forming the gate electrodes, thereby forming a highly oxy-nitrided region. In this manner, in a semiconductor device in which there coexist a MISFET having a thin gate insulation film and a MISFET having a thick gate insulation film, hot carrier reliability of the MISFET having the thick gate insulation film is improved.