In a nonvolatile semiconductor memory device provided with memory cell transistors, each of the memory cell transistors has a tunnel insulating film, a floating gate electrode, an inter-electrode insulating film, and element isolation insulating films respectively. The floating gate electrode on the tunnel insulating film is provided with a first floating gate electrode and a second floating gate electrode formed sequentially from the bottom, the second floating gate electrode being narrower in a channel-width direction than the first one. Levels of upper surfaces of the element isolation insulating films and the first floating gate electrode are the same. The inter-electrode insulating film continuously covers the upper and side surfaces of the floating gate electrode and the upper surfaces of the element isolation insulating films, and is higher in a nitrogen concentration in a boundary portion to the floating gate electrode than in boundary portions to the element isolation insulating films.