This invention has a purpose to provide an insulated gate-type semiconductor device and its manufacturing method in which a decrease in gate insulation dielectric strength voltage and a reduction in manufacturing costs are both achieved. First, (a) a CZ bulk substrate is prepared. Next, (b) P− diffused layer and N+ diffused layer 31 are formed by executing processes such as ion implantation and thermal diffusion treatment. Further (c) a gate trench is formed by reactive ion etching. Next, (d) a gate insulation film containing carbon of 1.0×1018 atoms/cm3 is formed on the wall face of a gate trench according to a CVD method and, annealing is subsequently performed. As a consequence, a low defect area is formed in the vicinity of an interface between the CZ bulk substrate and the gate insulation film.