A semiconductor device according to the invention includes p-type well region 3 and n+ source region 4, both formed selectively in the surface portion of n− drift region 2; trench 6 in contact with n+ source region 4 and extending through p-type well region 3 into n− drift region 2; field plate 8 formed in trench 6 with first insulator film 7 interposed between the trench 6 inner surface and field plate 8; gate electrode 10 formed in trench 6 with second insulator film 9 interposed between the trench 6 side wall and gate electrode 10, gate electrode 10 being formed above field plate 8; first insulator film 7 being thicker than second insulator film 9; and n−− lightly doped region 21 in n− drift region 2, n−− lightly doped region 21 crossing under the bottom surface of trench 6 from the corner portion thereof, n−− lightly doped region 21 covering the bottom surface of trench 6. The semiconductor device according to the invention and the method of manufacturing the semiconductor device according to the invention facilitate lowering the ON-state voltage, preventing the breakdown voltage from lowering, lowering the gate capacitance, and reducing the manufacturing costs.