Method of manufacturing semiconductor device

To provide a semiconductor device having improved performance while improving the throughput in the manufacturing steps of the semiconductor device. An insulating film portion comprised of first, second, third, fourth, and fifth insulating films is formed on a semiconductor substrate. The second insulating film is a first charge storage film and the fourth insulating film is a second charge storage film. The first charge storage film contains silicon and nitrogen; the third insulating film contains silicon and oxygen; and the second charge storage film contains silicon and nitrogen. The thickness of the third insulating film is smaller than that of the first charge storage film and the thickness of the second charge storage film is greater than that of the first charge storage film. The third insulating film is formed by treating the upper surface of the first charge storage film with a water-containing treatment liquid.