An insulated gate semiconductor device, includes an isolating structure shaped in a circulating section along the periphery of a semiconductor substrate to isolate that part from an inside device region, a peripheral diffusion region of the semiconductor substrate located outside the isolating structure, a plurality of cell structures defined in the inside device region and divided in segments by insulated trench-shaped gates to have a base region covered with an emitter region in its upper surface, a collector region, and an emitter electrode electrically connected to the emitter region and the base region, a dummy base region contiguous to the cell structures and configured as a base region that has its upper surface left without the emitter region connected to the emitter electrode, an inner region defined in and insulated from the dummy base region, and a connection part to electrically connect the inner region to the emitter electrode.