a manufacturing method of semiconductor device includes : forming a nitride film above a silicon substrate including a first region and a second region which respectively correspond to an outside of a memory cell region and the memory cell region ; forming trenches reaching from the nitride film to the silicon substrate ; retreating the nitride film such that widths of the trenches at the nitride film become wider ; forming a buried oxide film to be buried in the trenches after the retreating ; polishing the buried oxide film with the nitride film being used as a stopper ; removing the nitride film after the polishing ; implanting impurity after the removing ; forming gate electrodes after the implanting ; and implanting impurity after the forming the gate electrodes .