A plasma processing apparatus adapted to reduce non-uniformity of plasma distribution in a process chamber and to adjust the plasma distribution to “centrally high density”, “circumferentially high density”, or “uniform density” in accordance with a desired etching process, a process chamber; a radio frequency power source; a rectangular waveguide; and a circular waveguide connected to the rectangular waveguide, in which the rectangular waveguide includes an upper rectangular waveguide and a lower rectangular waveguide formed by vertically dividing the rectangular waveguide; and a cutoff section which cuts off the microwave frequency power and which has a dielectric body. The circular waveguide includes an inner waveguide connected to the upper rectangular waveguide and formed inside; and an outer waveguide connected to the lower rectangular waveguide and formed on an outer side of the inner waveguide. The cutoff section has a width narrower than those of the rectangular waveguides except the cutoff section.