To provide the manufacturing method of a semiconductor device using an STI of excellent transistor characteristics.

The manufacturing method of a semiconductor device comprises (a) a process of forming a stopper layer of Chemical Mechanical Polishing on a semiconductor substrate surface; (b) a process of forming a device separation trench in the stopper layer and in the semiconductor substrate; (c) a process of depositing a nitride film so as to cover a surface in the trench; (d) a process of depositing a first oxide film using high density plasma oxidation so as to embed at least a lower portion of the trench on which the nitride film is deposited; (e) a process of washing out the first oxide film on the trench side wall with dilute hydrofluoric acid, and leaving the nitride film behind by at least a part of the thickness thereof; (f) a process of depositing a second oxide film with high density plasma oxidation so as to embed the trench after washed out; and (g) a process of removing the oxide film on the stopper layer with Chemical Mechanical Polishing.