Abstract: In a plasma processing apparatus including a plasma chamber having a narrow window, and a rectangular waveguide for coupling with the plasma chamber, the rectangular waveguide has a long slot disposed in an E-plane thereof so as to oppose the narrow window of the plasma chamber and to extend along a waveguide-axis direction of the rectangular waveguide. There is further provided at least two long slots disposed in at least one rectangular waveguide, and the longitudinal length of each long slot is set to 1/2 or more of a free-space wavelength of the microwave. Further, the long slots are disposed so as to be parallel to each other such that adjacent long slots are shifted from each other by (2n-1)/4 of the free-space wavelength of the microwave in the waveguide-axis direction of the rectangular waveguide, where n is a natural number.