A semiconductor device manufacturing method includes forming a first insulating film on a semiconductor substrate containing silicon, the first insulating film having a first dielectric constant and constituting a part of a tunnel insulating film, forming a floating gate electrode film on the first insulating film, the floating gate electrode film being formed of a semiconductor film containing silicon, patterning the floating gate electrode film, the first insulating film, and the semiconductor substrate to form a first structure having a first side surface, exposing the first structure to an atmosphere containing an oxidizing agent, oxidizing that part of the floating gate electrode film which corresponds to a boundary between the first insulating film and the floating gate electrode film using the oxidizing agent, to form a second insulating film having a second dielectric constant smaller than the first dielectric constant and constituting a part of the tunnel insulating film.