A trench (21) is formed in a silicon substrate (1) on which an underlying oxide film (2) and a silicon nitride film (3) are formed. Then, a silicon oxide (11) is deposited by an HDP-CVD method to fill the trench (21) with the oxide. Further, a resist (41) including a second resist portion (42), and a resist (43) are formed. The silicon oxide film (11) that is not covered with the resists (41) and (43), is removed by dry etching. Etch selectivity of the silicon oxide film (11) to the stopper film (3) is not less than a value (2(c−a)/d) obtained by dividing twice a value (c−a) which is obtained by subtracting an alignment margin (a) from the maximum film thickness (c) of the silicon oxide film (11), by the film thickness (d) of the stopper film (3). The resists (41) and (43) are then removed, and the residual silicon oxide film (11B, 11DC, 11DE, 11FE) is polished and removed by the CMP method. This forms a trench type element isolation with no depression at its edge portion.