The present invention improves the performance of a semiconductor device. In a manufacturing method of a semiconductor device, sacrificial oxide films are formed over the side surface of a control gate electrode formed in a memory cell region, the surface of a cap insulating film formed in the memory cell region, and the surface of the part, which remains in a peripheral circuit region, of an insulating film. The step of forming the sacrificial oxide films includes the steps of: oxidizing the side surface of the control gate electrode by a thermal oxidation method; and oxidizing the surface of the cap insulating film and the surface of the part, which remains in the peripheral circuit region, of the insulating film by an ISSG oxidation method.