PURPOSE: To prevent the excessive diffusion of impurity in the semiconductor element by forming a polysilicon layer becoming a wire on a gate oxide film formed on a semiconductor substrate, then oxidizing the surface of the polysilicon layer and thereafter diffusing the impurity.

CONSTITUTION: A gate oxide film 12 is formed on the P type silicon substrate 11. Then, a polysilicon layer 13 becoming the wire is formed and heated at approx. 1,000°C. Thus, the oxide film 14 is formed on the surface of the polysilicon layer 13. Thereafter, POCl3 is sprayed as indicated by an arrow to diffuse the phosphorus in the layer 13. Since an oxide film is formed on the surface in this manner, it is diffused in the oxide film 14 and then diffused in the layer 13 with the result that the pipe diffusion phenomenon does not occur to the layer 13. Accordingly, it can prevent the diffusion from reaching the boundary of the substrate through the gate oxide film.