A semiconductor device manufacturing method includes forming a gate insulating film on a semiconductor substrate; forming, on the gate insulating film, a multilayered structure including at least a polysilicon layer and a metal layer containing a refractory metal; forming a gate electrode by etching the multilayered structure; and performing a plasma process by a plasma processing apparatus, which is configured to supply microwaves into a process chamber from a planar antenna including a plurality of slots and thereby to generate plasma, at a process pressure of 133.3 to 1,333 Pa and a process temperature of 250 to 800° C. by using a process gas containing at least hydrogen gas and oxygen gas, thereby selectively oxidizing the polysilicon layer in the gate electrode.