A microwave-excited plasma processing apparatus shows a wide pressure range and a wide applicable electric power range for normal electric discharges as a result of using slits cut through a rectangular waveguide and having a profile that allows the electric field and the magnetic field of microwave to be formed uniformly right below the microwave introducing window below an microwave antenna. The microwave-excited plasma processing apparatus is characterized by having four elliptic slits cut through the wall of the rectangular waveguide that is held in contact with the microwave introducing window of the top wall of the vacuum chamber, the four elliptic slits being arranged respectively along the four sides of a substantial square.