The rates of electrochemical reactions were clarified in a CH4-H2O system at the interface of a porous Pt electrode/Y2O3-stabilized ZrO2 (YSZ) electrolyte between 873 and 1173 K to elucidate the kinetics of the anode reaction of solid oxide fuel cells (SOFCs). The dominant electrochemical reaction was found to be the redox process of H-2/H2O, where H-2, C, CO, and CO2 were formed without a current by the chemical reactions in a CH4-H2O system. The partial electrochemical reaction rates of H-2, CO, C, and CH4 were determined. The rate of the electrochemical reaction of CO/CO2 in a CH4-H2O system is larger than that in a CO-CO2 system under anodic polarization at 873 and 973 K. This means both the efficiency and the rate of the utilization of fuels on SOFCs are increased.