To realize a semiconductor device having a power MOSFET satisfying both a low conduction resistance and a high junction breakdown voltage by a simple and easy manufacturing method. Over an n-type substrate, a p-type epitaxial layer of a low concentration is formed, and, in an active part, a plurality of active regions is defined by a plurality of trenches that is formed in the epitaxial layer and extends in a first direction with first intervals in a second direction orthogonal to the first direction. In the epitaxial layer between the adjacent trenches, an n-type diffusion region that functions as a drain offset layer is formed, and, in the epitaxial layer between a side wall of the trench and the n-type diffusion region, a p-type diffusion region connected with a channel region (the p-type diffusion region) is formed, to constitute a super junction structure. Further, by forming an n-type diffusion region in the epitaxial layer, having a prescribed width from a side wall of a trench lying in the end part of the active part toward an outer periphery part, to achieve the improvement of a drain breakdown voltage.