The manufacturing method of the semiconductor device includes a step of forming the gate dielectric film GI2 and the polysilicon layer PS2 on the main surface SUBa of the semiconductor substrate SUB, a step of forming the isolation trench TR in the semiconductor substrate SUB through the polysilicon layer PS2 and the gate dielectric film GI2, a step of filling the isolation trench TR with the dielectric film, and then a step of polishing the dielectric film to form the element isolation film STI in the isolation trench TR. Further, a method for manufacturing a semiconductor device comprises etching the element isolation film STI to retract the upper surface STIa of the element isolation film STI, then further depositing a polysilicon layer on the polysilicon layer PS2 to form a gate electrode using an anisotropic dry etching method.