The rate processes of electrochemical reactions were clarified in a CH4-H2O system at the interface of a porous Pt electrode/Y2O3-stabilized ZrO2 electrolyte. Direct-current polarization measurements and ac impedance spectroscopy were made with gas analysis before and after the reaction at 1073 K. We proposed an analytical method to determine the rates of electrochemical reactions taking place in parallel. When the ratio p(H2O)/p(CH4) of the inlet gas was close to zero, the observed relationship between the polarization current and electrode potential was interpreted by the electrochemical oxidation processes of H2, CO, C, and CH4 in parallel using the proposed method. For example, the ratio of the oxidation rates for C/CO/CH4/H2 is 1/1.3×10/1.9×102/2.8×103 at E = -600 mV vs. air. This result was obtained under very low CH4 concentration. The estimated oxidation rates of H2 and CO as functions of the electrode potential were described by the model proposed by Mizusaki et al. for the reaction of H2-H2O and CO-CO2.