A plasma device for uniform processing of large-size wafers. The device comprises a processing container (4) that is a bottomed cylinder with an upper opening covered with a quartz plate (8), the cylinder including a processing stage (10) for holding a wafer (W); a microwave device (50) for supplying TE11-mode microwave; a cylindrical waveguide (52) including a waveguide space, connected with the microwave device (50) on one end, and extending toward the quartz plate (8); a radial waveguide box (54) including a waveguide space, connected with the other end of the waveguide (52), spreading radially outward in the shape of a flange, and falling toward the quartz board (8) to serve as a side wall; and a disk-like slot antenna (60) arranged along the quartz board (8) to cover a lower opening of the waveguide box (54). A circular polarization converter (56) can be provided to rotate microwave on the axis of the cylindrical waveguide (52) and send it to the waveguide box (54).