Method for manufacturing a nonvolatile semiconductor storage device

1. Technical Field of the Invention According to the present invention, a method for manufacturing a nonvolatile semiconductor storage device is provided in which an element separating layer and first gate insulating layer are respectively formed onto a silicon base, followed by layering and patterning of a first polysilicon layer. A second gate insulating layer for forming an ONO structure and a second polysilicon layer are then sequentially formed and patterned. After formation of a gate oxide layer, a third polysilicon layer is layered and patterned to form a gate electrode. The second and first polysilicon layers are then patterned to respectively form control gate and floating gate.