An improvement is achieved in the performance of a semiconductor device having a nonvolatile memory. A first memory cell includes a first control gate electrode and a first memory gate electrode which are formed over a semiconductor substrate to be adjacent to each other. A second memory cell includes a second control gate electrode and a second memory gate electrode which are formed over the semiconductor substrate to be adjacent to each other. A width of a sidewall spacer formed on a side of the second memory gate electrode opposite to a side thereof where the second memory gate electrode is adjacent to the second control gate electrode is smaller than a width of another sidewall spacer formed on a side of the first memory gate electrode opposite to a side thereof where the first memory gate electrode is adjacent to the first control gate electrode. A threshold voltage of a first memory transistor including the first memory gate electrode in a neutral state is different a threshold voltage of a second memory transistor including the second memory gate electrode in the neutral state.