Mutual diffusion of impurities in a gate electrode is suppressed near a boundary between an n-channel type MISFET and a p-channel type MISFET, which adopt a polycide's dual-gate structure. Since a gate electrode of an n-channel type MISFET and a gate electrode of a p-channel type MISFET are of mutually different conductivity types, they are separated to prevent the mutual diffusion of the impurities and are electrically connected to each other via a metallic wiring formed in the following steps. In a step before a gate electrode material is patterned to separate the gate electrodes, the mutual diffusion of the impurities before forming the gate electrodes is prevented by performing no heat treatment at a temperature of 700° C. or higher.