The present invention improves the production yield of a semiconductor device having nonvolatile memory cells of a split gate structure. The level difference of a lower layer resist film with which an end of a memory mat is covered is gentled, the uniformity of the thickness of a resist intermediate layer formed over the lower layer resist film is improved, and local thickness reduction or disappearance is prevented by, after forming a silicon oxide film and a silicon nitride film over each of selective gate electrodes formed in a memory cell region of a semiconductor substrate, removing the silicon oxide film and the silicon nitride film over the selective gate electrode located on the outermost side (a dummy cell region) of the memory mat in the gate length direction.