The internal reforming operation is one of the advantages for solid oxide fuel cells. In this study, we aimed to elucidate the influence of anode thickness in the electrolyte-supported cells on the performance stability and deterioration behavior with a supply of humidified methane fuel. The increase in the anode thickness enhanced the open circuit voltage, indicating the enhancement of methane conversion in the thick electrode layer. The cells with the 20- and 30-μm thick anodes attained stable performance at 0.2 A/cm2. In contrast, for the anode in 10-μm thickness, the terminal voltage gradually decreased and suddenly oscillated. This behavior corresponded to the gradual increase in ohmic resistance and the drastic change in the anodic overpotential. From SEM observation and Raman analysis, the deposited amorphous carbon obstructed the pores in this anode. Thus, it was concluded the performance deterioration was induced by the deposition of low-conductive carbon species and the concomitant inhibition of gas diffusion.