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Large-area and uniform plasma production is of crucial importance for achieving large-scaled material processing. In particular, with the progress of memory sizes of microprocessors, the uniformity of plasma parameters over a large-area is an essential problem for high speed and high quality process. A rotating mode radial line slot antenna (RLSA) with densely arrayed slots has already been proposed for plasma processing system and the basic characteristics were confirmed. In our previous work, however, the feeding circuit using cylindrical waveguide to excite the rotating mode was very large and bulky. In order to make the most of the advantages of small and low profile apparatus by using RLSAs, improvement of the size of feeding circuit as well as of the plasma chamber is required. Moreover, enhancement of the rotational symmetry by optimizing the feeding circuit is also required. This paper proposes a novel feeding circuit using a cavity resonator in plasma processing RLSAs. The rotational symmetry of the field intensity generated in a parallel plate as well as the input impedance is evaluated by using a mode matching technique. The rotational symmetry with the amplitude deviations of less than 2 dB and good matching with a coaxial waveguide is realized at the same time. The design of the reflection characteristics is confirmed by Ansoft-HFSS.