As one of the key technologies in the development of a direct internal-reforming solid oxide fuel cell, catalytic activity and stability of a Ni-YSZ-CeO2 anode on a zirconia electrolyte for the steam reforming of methane was investigated by experiments using a differential fuel cell reactor. The effects of the partial pressure of CH4, H2O and H-2, and temperature as well as the electrochemical oxidation on the catalytic activity were analyzed. It was found that the catalytic activity of the Ni-YSZ-CeO2 anode was higher than that of the Ni-YSZ reported especially at low temperature. A deterioration of the catalytic activity of the anode was observed at low P-H2 and high P-H2O atmosphere, and also at high current densities. This might be caused by the oxidation of the Ni surface by H2O in the reaction gas and that produced by the anodic reaction. A rate equation for a fractional function for the steam reforming on open circuit was also proposed.