Cutting work is performed on an n-semiconductor substrate (1) with an inverted trapezoid-shaped dicing blade to form grooves to be a second side walls (7). Bottom portions of the grooves are contacted with a p-diffusion layer (4) which is formed on a first principal plane (2) (front face) of the n-semiconductor substrate (1), so that the p-diffusion layer (4) is not cut. Then in the second side walls (7), a p-isolation layer (9) connected to a p-collector layer (8) and the p-diffusion layer (4) is formed. Since the p-diffusion layer (4) is not cut, a glass support substrate for supporting a wafer, and expensive adhesive, are not required, and therefore the p-isolation layer (4) can be formed at low cost.