A high-breakdown-voltage semiconductor device comprises a high-resistance semiconductor layer, trenches formed on the surface thereof in a longitudinal plane shape and in parallel, first regions formed on the semiconductor layer to be sandwiched between adjacent ones of the trenches and having an impurity concentration higher than that of the semiconductor layer, a second region having opposite conductivity to the first regions and continuously disposed in a trench sidewall and bottom portion, a sidewall insulating film disposed on the second region of the trench sidewall, a third region disposed on the second region of the trench bottom portion and having the same conductivity as and the higher impurity concentration than the second region, a fourth region disposed on the back surface of the semiconductor layer, a first electrode formed on each first region, a second electrode connected to the third region, and a third electrode formed on the fourth region.