Reliability of a semiconductor device is improved. A resist pattern having an opening in a first region where a memory transistor is formed and covering other regions is prepared. Next, by ion implantation using this resist pattern as a mask, a channel region is formed in a surface of a semiconductor substrate in the first region, and a nitrogen-introduction portion is formed inside the channel region. Next, the resist pattern is removed. Then, a gate insulating film having a charge storage layer is formed on the semiconductor substrate in the first region, and a gate electrode is formed on the gate insulating film.