Activity and carbon deposition behavior were investigated on the catalyst for CO2 reforming of methane under pressurized conditions. A Ni0.03Mg0.97O solid solution catalyst with low surface area (similar to 4 m(2)/g) exhibited a lower carbon formation rate than other NiO-MgO solid solution and MgO-supported Pt catalysts. It is suggested that the suppression of Ni aggregation is important for the decrease of the carbon formation rate. The additive effect of Sn, Ge, and Ca to a Ni-0.03-Mg0.97O solid solution was investigated, and it was found that Sn is an effective component for the decrease of the carbon amount, which is especially formed by methane decomposition.