PURPOSE: To form the transistor in the structure having the boundary between source.drain diffused resistor and a substrate in a gentle distribution of concentration in excellent reproducibility by a method wherein, after the removal of a gate electrode sidewall, the whole body is high temperature heat-treated to activate implanted impurities in the state of a substrate surface covered with an outer diffusion preventive insulating film of the impurities in even thickness.

CONSTITUTION: A channeling preventive CVD-SiO2 film 9 and an SiO2 film sidewall 8 are removed by wet-etching step using, e.g. fluoric acid solution. Next, after the formation of an outer diffusin preventive CVD-SiO2 film 12 on the whole surface of this substrate, the whole body is high temperature annealed in an inert gas atmosphere to activate P+ in low concentration P+ implanted region 107 and As+ in high concentration As- implanted region 110 so that n- type low concentration offset regions 7S, 7D and an n+ type high concentration source region 10S, an n+ type high concentration drain region 10D may be formed. Through these procedures, a short channel MOS transistor having the boundary between the source-drain diffused regions and the substrate beneath 8 gate electrode in a gentle distribution of concentration to relieve the electric field in that part can be formed in excellent reproducibility.