A semiconductor device includes: a semiconductor substrate; an element region having a semiconductor element including an impurity layer and a trench, wherein the impurity layer is disposed in the trench, and wherein the trench is disposed on a main surface of the substrate; and a field region disposed around the element region. The trench is an aggregation of a plurality of stripe line trenches so that the element region has a polygonal shape. The field region includes a dummy trench disposed along with one side of the polygonal shape on a periphery of the element region. The dummy trench has a width and a longitudinal direction, which are equal to those of the trench. The field region further includes an impurity layer disposed in the dummy trench.