Solid oxide fuel cells (SOFCs) are potentially driven by directly feeding humidified methane to the anodes when steam reforming of methane proceeds using nickel in the anode as a catalyst. The advantage is that the heat released from the electrochemical reaction, which is usually wasted, can be effectively utilized for the reforming reaction. However, the reforming reaction, which is a strong endothermic reaction, can proceed rapidly near the fuel inlet, resulting in a large temperature gradient on a cell. We recently reported that mixing ammonia with humidified methane allows the methane reforming reaction to proceed gradually at the Ni–YSZ electrode, a typical anode material of SOFCs,. As a result, the steep temperature gradient is mitigated. In this study, we experimentally investigate the effect of catalyst supports on the reaction characteristics of the methane–ammonia mixed gas in the nickel-based catalyst. In the experiment, inlet gases with various compositions are supplied to catalysts with different catalyst support materials, i.e., Ni–YSZ, Ni–GDC, and Ni–Al2O3. It is found the steam methane reforming reaction is suppressed by adding ammonia in all the catalysts analyzed, while the extent of suppression depends on the catalyst support. The suppression effect is more significant in the Ni–YSZ, Ni–GDC, and Ni–Al2O3, in that order. The porous microstructure of the catalysts and the adsorption/desorption isotherms are also obtained and analyzed. From these results, it is suggested that the suppression effect can be associated with the amount of nickel–pore contact surface area and the adsorption property of the support materials.