Disclosed is provides a plasma processing apparatus that processes a workpiece. The plasma processing apparatus includes: a processing container configured to accommodate the workpiece; a coaxial waveguide configured to transmit microwaves generated in a microwave generator; and a slow wave plate configured to adjust a wavelength of the microwaves transmitted from the coaxial waveguide and to introduce the microwaves into the processing container. A lower end portion of an inner conductor of the coaxial waveguide has a tapered shape of which a diameter increases downwardly, the slow wave plate has an annular shape in a plan view, and the inner surface of the slow wave plate encloses the lower end portion of the inner conductor and is located more outside than an inner surface of an outer conductor of the coaxial waveguide in a radial direction.