PURPOSE: To realize a semiconductor nonvolatile storage device and its manufacturing method wherein the increase of a voltage necessary for writing and the irregularity of data writing characteristics can be restrained, the influence upon data writing characteristics which is caused by the deterioration of a gate insulating film as the result of repeated operation is little, and reliability can be improved.

CONSTITUTION: In a DINOR type semiconductor nonvolatile storage device, the structure of a drain diffusion layer 3a is constituted as an so-called asymmetric memory cell wherein the overlap part with a floating gate 5 is larger than the structure of a source diffusion layer 2, and the diffusion concentration of the part is set high. Thereby the stretch of the diffusion layer from the source side can be restrained, and the enlargement of short channel phenomenon can be restricted only in the drain side.