A nonvolatile semiconductor memory device with improved writing characteristics. The memory device has memory cell transistors arranged in rows and columns. The memory cell transistors belonging to the same column share a source region and a drain region, and a channel region is disposed between the source and drain regions. The interval between the source and drain regions is the isolation width. Each of the memory cell transistors has a floating gate electrode disposed on the channel region with a first gate insulating film and a control gate electrode disposed on the floating gate electrode with a second gate insulating film. The floating gate electrode does not have a constant width and has a portion narrower than the isolation width which is free from the floating gate electrode. The narrower portion is typically formed as a constricted portion of the floating gate electrode.