In a plasma processing apparatus using a circularly polarized wave, in order to reduce an influence caused by a reflected wave and to efficiently utilize a circularly polarized wave for a plasma process, the plasma processing apparatus includes a microwave source configured to generate a microwave, a plasma processing chamber configured to process a processing target disposed therein by using plasma generated by the microwave, and a waveguide portion that includes a rectangular waveguide connected to the microwave source and a circular waveguide connected to the plasma processing chamber, and the plasma processing apparatus is further provided with, inside the circular waveguide, a reflected wave generator configured to generate a reflected wave that cancels a reflected wave propagating in the circular waveguide from a plasma processing chamber side in a state where the plasma is generated inside the plasma processing chamber by the microwave.

A plasma generator includes: a microwave-generating source that generates a microwave; a cylindrical plasma generation tube through which a gas to be plasmatized by the microwave flows; a waveguide that transmits the microwave to the plasma generation tube; a matching device provided to the waveguide between the microwave-generating source and the plasma generation tube; a cooling jacket that is provided on an outer peripheral surface of the plasma generation tube for cooling; and a cylindrical casing that houses the plasma generation tube and the cooling jacket. The cooling jacket includes: three slits that extend along a traveling direction of the gas flowing through the plasma generation tube, or along a direction inclined with respect to the traveling direction, and through which the microwave passes toward the plasma generation tube; and a cooling channel that is provided between the slits adjacent to each other, and through which a cooling fluid flows.