To provide a semiconductor device manufacturing method by which a semiconductor device using trench element separation can be manufactured at a high yield, by preventing the occurrence of a short-circuiting failure, etc., between adjacent gate electrodes, etc.

A semiconductor device manufacturing method includes a step of coating the main surface of a semiconductor substrate 41 with a laminated film 46 formed by successively laminating a first insulating film (thermally oxidized silicon film) 42, a first stopper film (silicon nitride film) 43, a second insulating film (silicon oxide film) 44, and a second stopper film (silicon nitride film) 45 upon another, a step of forming grooves 49 for separating an active area 48 into the substrate 41 from the main surface carrying the laminated film 46 and buried insulating layers (silicon oxide layers) 52 on the whole surface of the substrate 41 so as to fill up the grooves 49, and a step of leaving the insulating layers 52 in the grooves 49 by flattening the layers 52 to the second stopper film 45 by chemical mechanical polishing. The method also includes a step of again polishing the upper surfaces of the insulating layers 52 to the first stopper film 43 by chemical mechanical polishing after removing the second stopper film 45.