Disclosed herein is a nonvolatile semiconductor memory device, including a memory transistor. The memory transistor has a channel formation region defined between two source and drain regions formed on a semiconductor substrate a bottom insulating film, a charge storage film and a top insulating film formed in order at least on the channel formation region, the charge storage film having a charge storage function, and a gate electrode formed on the top insulating film. The bottom insulating film is formed from a plurality of films containing nitrogen such that the content of nitrogen of a lowermost one of the films which contacts with the channel formation region and an uppermost one of the films which contacts with the gate electrode is higher than that of the other one or ones of the films which exist between the uppermost and lowermost films.