According to a fabrication method for an element isolation structure section, that is, STI, of the present invention, by differing the etching rate of material to be embedded in a narrow-width, that is, a small area trench section (first trench section) formed in a small isolation area, from the etching rate of a material to be embedded in a wide-width (plane shape of larger area) trench section (second trench section) formed in a large isolation area, in the etching step, dishing (recessing) that inevitably occurs in a CMP step can be reduced. Therefore, a STI having a higher level of flatness can be formed. As a result, by simple steps, deterioration of the electrical characteristics of elements that are element-isolated by STI can be reduced. That is to say, not only STI having excellent electrical characteristics, but also a semiconductor device provided with such STI, can be provided at a good level of production yield.Moreover, according to the fabrication method for STI of the present invention, since excellent in-plane uniformity can be achieved, further miniaturization of the semiconductor device fabrication process can be supported.