An improvement is achieved in the performance of semiconductor device including a nonvolatile memory. In a split-gate nonvolatile memory, between a memory gate electrode and a p-type well and between a control gate electrode and the memory gate electrode, an insulating film is formed. Of the insulating film, the portion between the lower surface of the memory gate electrode and the upper surface of a semiconductor substrate has silicon oxide films, and a silicon nitride film interposed between the silicon oxide films. Of the insulating film, the portion between a side surface of the control gate electrode and a side surface of the memory gate electrode is formed of a silicon oxide film, and does not have the silicon nitride film.