Abstract: A plasma processing apparatus comprises a processing container, a waveguide tube for guiding microwaves generated by a microwave generator, and a flat antenna member connected to the wave guide and disposed in the container to face a semiconductor wafer supported in the container. The antenna includes a plurality of short slits concentrically or spirally arranged in the antenna. The slits are spaced apart in the widthwise direction at intervals of 5% to 50% of a guide wavelength of the microwave, and each of the slits has a length of +30% of the guide wavelength centered with respect to half of the guide wavelength.