PURPOSE: To restrain the occurrence of OSF containing specified number or more of atomic pores in a silicon semiconductor substrate.

CONSTITUTION: A wafer 12 arranged on a silicon board 14 is inserted into a silicon tube 11 at a rate of 5cm/min and then it is heat treated in a heat treating furnace 10 of oxygen atmosphere at 900-1000°C thus depositing an oxide on the surface of the wafer 12. Thereafter, the wafer 12 is taken out of the silicon tube 11 at a rate of 5cm/min. Subsequently, the wafer 12 is inserted again into the silicon tube 11 and heat treated in the heat treatment furnace 10 of argon atmosphere for 20-120min at 1100-1200°C thus generating 1016/cm3 or more of atomic pores in the wafer 12. The wafer 12 is then taken out of the silicon tube 11 and cooled or quenched at a rate of 800°C/min or above by blowing an inert gas thus confining 1016/cm3 or more of atomic pores in the wafer 12.