To provide a semiconductor device with improved reliability which includes a metal silicide layer formed by a salicide process. After forming gate electrodes, an n+-type semiconductor region, and a p+-type semiconductor region for a source or drain, a Ni1−xPtx alloy film is formed over a semiconductor substrate. The alloy film reacts with the gate electrodes, the n+-type semiconductor region, and the p+-type semiconductor region by a first heat treatment to form a metal silicide layer in a (Ni1−yPty)2Si phase. At this time, the first heat treatment is performed at a heat treatment temperature where a diffusion coefficient of Ni is larger than that of Pt. Further, the first heat treatment is performed such that a reacted part of the alloy film remains at the metal silicide layer. This results in y>x. Then, after removing the unreacted part of the alloy film, the metal silicide layer is further subjected to a second heat treatment to form a metal silicide layer in a Ni1−yPtySi phase.