The invention provides a nonvolatile semiconductor storage device and a technology of a manufacturing method, wherein the technology increases charge retain characteristic of the nonvolatile semiconductor storage element using a metal oxide film as a charge accumulation film through restraining mutual dispersion of the metal oxide film and an insulation film gripped up and down. The insulation film (4) for keeping the charge of a memory cell MC1 is composed of a lower insulation film (4a), a charge accumulation film (4c) composed of the metal oxide film and a laminate film obtained by an upper insulation film (4e) formed in turn on a channel region side of a semiconductor substrate (1), through performing plasma nitrogen treatment on the lower insulation film (4a), a nitridation region (4b) having more than 1 atom% nitrogen concentration and a peak value is formed on an upper side of the lower insulation film (4a), wherein the thickness of the nitridation region (4b) is in the range of 0.5 nm to 1.5 nm.