Abstract: In a plasma processing apparatus including a plasma chamber having a strip-shaped rectangular window having a longitudinal length formed in a side wall of the plasma chamber, an object to be processed is provided within the plasma chamber so as to be close to the window. The plasma processing apparatus includes a coupling rectangular waveguide having a strip-shaped rectangular slit formed in an E-plane thereof so as to extend in a direction of an axis thereof, wherein the coupling rectangular waveguide is provided so that the direction of the axis of the coupling rectangular waveguide is parallel to the longitudinal direction of the window of the plasma chamber. A coupling member made of a material for propagating a microwave is mounted between the slit of the coupling rectangular waveguide and the window of the plasma chamber, and electromagnetically couples the slit with the window in an airtight state.