A semiconductor device having a high degree of reliability is provided. A second object of the invention is to provide a semiconductor device of high yield. The semiconductor includes a silicon substrate, a gate dielectric film formed on one main surface of the silicon substrate, a gate electrode formed by being stacked on the gate dielectric film and a diffusion layer containing arsenic and phosphorus. Both of the concentration of the highest concentration portion of arsenic and the concentration of the highest concentration portion of phosphorus are each at 10.sup.26 atoms/m.sup.3 or more and 10.sup.27 atoms/m.sup.3 or less, and the depth of the highest concentration portion of phosphorus from the surface of the silicon substrate is less than the depth of the highest concentration portion of arsenic.