In a MIS-type semiconductor device having a trench gate structure, a withstand voltage is ensured without changing the thickness of a drift layer and on-resistance can be reduced without applying a high gate drive voltage. The lower half of a trench extending through a p-base region into an n-drift region is filled with a high-permittivity dielectric having a relative permittivity that is higher than that of a silicon oxide film, preferably a silicon nitride film, and an insulated gate structure including a gate insulator and a gate electrode is fabricated on the high-permittivity dielectric. The depth d2 of the deepest portion of the high-permittivity dielectric is designed to be deeper than the depth d1 of a depletion layer in the semiconductor region away from the high-permittivity dielectric.