A nonvolatile semiconductor memory device comprises: element isolation insulating films formed in a semiconductor substrate in a first direction; and element regions formed in a region sandwiched by the element isolation insulating film, with MONOS type memory cells. The MONOS type memory cell comprises: a tunnel insulating film disposed on the element region; a charge storage film disposed continuously on the element regions and the element isolation insulating films. The charge storage film comprises: a charge film disposed on the element region and having a certain charge trapping characteristic; and a degraded charge film disposed on the element isolation insulating film and having a charge trapping characteristic inferior to that of the charge film. The degraded charge film has a length of an upper surface thereof set shorter than a length of a lower surface thereof in a cross-section along the first direction.