A plasma processing apparatus includes a plasma processing chamber and a source of microwaves. The microwaves are introduced to the processing chamber by a slotted annular waveguide having inner and outer arc-shaped slots. The distance between the centerline of the inner and outer arc-shaped slots is set to be an even multiple of a half wavelength of a microwave surface wave propagating along a surface of a dielectric window of the chamber. A distance between the centerline of the outer arc-shaped slot and an outer periphery of the dielectric window is set to be an odd multiple of the half wavelength of the microwave surface wave.