An improvement is achieved in the performance of a semiconductor device having a nonvolatile memory. A memory cell of the nonvolatile memory includes a control gate electrode formed over a semiconductor substrate via a first insulating film and a memory gate electrode formed over the semiconductor substrate via a second insulating film to be adjacent to the control gate electrode via the second insulating film. The second insulating film includes a third insulating film made of a silicon dioxide film, a fourth insulating film made of a silicon nitride film over the third insulating film, and a fifth insulating film over the fourth insulating film. The fifth insulating film includes a silicon oxynitride film. Between the memory gate electrode and the semiconductor substrate, respective end portions of the fourth and fifth insulating films are located closer to a side surface of the memory gate electrode than an end portion of a lower surface of the memory gate electrode. Between the memory gate electrode and the semiconductor substrate, in a region where the second insulating film is not formed, another silicon dioxide film is embedded.