The reliability of a semiconductor device is improved. A first gate electrode of a dummy gate electrode including silicon is formed over a semiconductor substrate. Then, by an ion implantation method, a semiconductor region for source or drain of MISFET is formed in the semiconductor substrate. Then, over the semiconductor substrate, an insulation film is formed in such a manner as to cover the first gate electrode. Then, the insulation film is polished to expose the first gate electrode. Then, the surface of the first gate electrode is wet etched by APM. then, the first gate electrode is removed by wet etching using aqueous ammonia. Thereafter, a gate electrode for MISFET is formed in a region from which the first gate electrode has been removed.