Microwaves propagated through the waveguide 30, a plurality of slots 31 and the dielectric members 33 in this order are supplied into the processing chamber U where they are used to excite a gas to plasma to be used to process a substrate G. Alumina 50 fills an area inside the waveguide 30 near an end surface C thereof, and the remaining area inside the waveguide is filled with Teflon 35. Since the alumina 50 has a smaller guide wavelength λg compared to the Teflon 35, the mechanical length measured from the end surface C of the waveguide 30 to the center of the closest slot is reduced compared to the mechanical length of a waveguide filled only with Teflon 35 while maintaining the physical characteristic length from the end surface C to the closest slot center at λg/4.