A semiconductor device comprises a trench-gate type field-effect transistor on a semiconductor substrate having a first main surface and a second main surface oppositely positioned in a thickness direction, wherein the trench-gate type field-effect transistor comprises a first semiconductor region at the first main surface side; a second semiconductor region at the second main surface; a semiconductor well region between the first semiconductor region and the second semiconductor region; a trench formed so as to protrude in a first direction intersecting the second main surface; a gate electrode formed on an inner surface of the trench via a gate insulating film, and a bottom of the gate electrode is in the first semiconductor region, and a well bottom has a well deep portion and a well shallow portion, and the well deep portion is in a region more distant from the gate insulating film compared to the well shallow portion.