An object of the present invention is to simplify manufacturing process of an n channel MIS transistor and a p channel MIS transistor with gate electrodes formed of a metal material. For its achievement, gate electrodes of each of the n channel MIS transistor and the p channel MIS transistor are simultaneously formed by patterning ruthenium film deposited on a gate insulator. Next, by introducing oxygen into each of the gate electrodes, the gate electrodes are transformed into those having high work function. Thereafter, by selectively reducing the gate electrode of the n channel MIS transistor, it is transformed into a gate electrode having low work function.