A silicon layer serving as a contact plug directly connected to a diffusion layer of a MOS transistor is provided. On a surface of an N.sup.- type diffusion layer in self-alignment with a silicon nitride layer spacer and a field oxide layer, an N.sup.+ type monocrystalline silicon layer formed by anisotropic selective epitaxial growth method is directly connected. The surface of the N.sup.+ type monocrystalline silicon layer is directly connected to an N.sup.+ type monocrystalline silicon layer formed by isotropic selective epitaxial growth.