A slot array antenna, consistent with the invention can include a power feeding waveguide for feeding microwave power and a plurality of rectangular radiating waveguides. The plurality of rectangular radiating waveguides are connected to a plurality of windows which are disposed along the longitudinal direction of the power feeding waveguide so as to guide the microwave power from the plurality of windows to the outside of the antenna. In each of the radiating waveguides, the interval "d" between the centers of gravity of slot pairs or slots is substantially the same as the wavelength m of the microwave in the rectangular radiating waveguide. A slot array antenna of the present invention can be used in a plasma processing apparatus to increase the plasma density in the plasma-processing chamber.