PURPOSE: To obtain a bipolar transistor characterized by a small area and a high gain, by controlling the amount of boron diffusion, and making the impurity concentration of an N-type impurity diffused region and N-type silicon higher than the impurity concentration of P-type silicon.

CONSTITUTION: An N-type diffused region 304, in which arsenic or antimony is diffused, and silicon dioxide films 302 and 303 having different thicknesses are formed on a P-type silicon crystal substrate 301. Boron ions are implanted into an opening part. Then, an N-type epitaxial crystal film is formed. N-type silicon 306, P-type silicon 307 and N-type silicon 308 are formed by the impurity diffusion from an N-type diffused region 305 in epitaxial growing and heat treatment. Then, the N-type silicon 308 becomes 308' and a polysilicon film 309 becomes 309, by a selective polishing method. An element-isolating insulating film 312 is formed by an element isolating technology. P-type high impurity concentration silicon 310 and N-type high impurity concentration silicon 311 for contacts are formed by impurity diffusion.