The microplasma system that uses a stripline resonator is a promising plasma system because, capitalizing on the resonance property of the plasma source, it can readily generate plasma under atmospheric conditions without the assistance of an additional matching network. However, precise determination of the resonance frequency before device manufacturing is very important in practice. Therefore, the evolution trends of the resonance frequency of the discharge have to be investigated, based on source impedance analysis, to get an insight into the discharge matching condition. In this paper, a means of determining the discharge source impedance, called conformal mapping, and its application to the microwave parallel stripline resonator discharge source is presented and discussed.