PURPOSE: To reduce the deterioration of element characteristics by a method wherein an impurity-implanted polycrystalline silicon layer is thinly oxidized to the full extent, a silicon layer with no impurities are implanted is formed thereon, and subsequently said silicon layer is oxidized.

CONSTITUTION: A floating gate 11 is formed by performing an etching on the impurity-implanted polycrystalline silicon layer 9 using a resist 10 as a mask. Subsequently, a source and drain 16 is formed by ion implanting of phosphorus or As. Then, a thermal oxidization 12 is performed on the surface of the gate 11 sufficiently thinner than the thickness of the desired oxide film, and after a polycrystalline silicon layer 13 has been formed without adding impurities thereon, a thermal oxidization 14 is performed on the whole surface of the layer 13, and an oxide film is formed on the desired floating gate. Subsequently, a control gate 15 is formed on the surface of the oxide film 14 in the same manner as the gate 11. As a result, a nonvolatile semiconductor memory storage having better retentive characteristics than the conventional products can be realized.