Abstract: Disclosed herein is a method to manufacture semiconductor memory devices that reduces the occurrence of excess deletion failure in a large number of non-volatile memory cells that may be electrically deleted at a time. The invented method also simplifies the manufacture process to improve the production throughput. A thin silicon film 11, a thin silicon oxide film 12, and a thick polycrystalline silicon film 13 that are laminated to comprise a floating gate electrode 4 are continuously formed in a heat treatment furnace having at least two gas inlets so that the film interfaces are not exposed to the atmosphere which usually contains contaminating impurities.