To provide a semiconductor device capable of having an ONO-film-configuring second oxide film with an optimized thickness. The semiconductor device has a semiconductor substrate having a first surface, a first gate insulating film placed on the first surface located in a first transistor formation region, and a second gate insulating film placed on the first surface located in a second transistor formation region. The first gate insulating film has a first oxide film, a first nitride film placed thereon, and a second oxide film placed thereon. The second oxide film includes a first layer and a second layer placed thereon. The height of the first surface in a region where the second insulating film is placed is lower than that in a region where the first gate insulating film is placed. The nitrogen concentration in the first layer is higher than that in the second layer.