A trench-type insulated-gate semiconductor device is disclosed that includes unit cells having a trench gate structure that are scattered uniformly throughout the active region of the device. The impurity concentration in the portion of a p-type base region, sandwiched between an n+-type emitter region and an n-type drift layer and in contact with a gate electrode formed in the trench via a gate insulator film, is the lowest in the portion thereof sandwiched between the bottom plane of n+-type emitter regions and the bottom plane of p-type base region and parallel to the major surface of a silicon substrate. The trench-type insulate-gate semiconductor device according to the invention minimizes the variation of the gate threshold voltage.