A Ni-Mg-O solid solution with low Ni content (Ni0.03Mg0.97O, atomic ratio) which was reduced at high temperature (> 1073 K) was found to be an active and stable catalyst for the steam reforming of methane in a steam to carbon ratio of 1.0. The reduced Ni0.03Mg0.97O catalyst showed higher activity and much higher stability than a commercial reforming catalyst (Ni/Al2O3-MgO). The content of metallic nickel on the catalyst surface reduced by hydrogen was only 0.17 mu mol/m(2) and the turnover frequency of the methane-steam reaction at 1023 K was 62 s(-1). The catalyst kept its activity for 60 h or more at 1123 K and a steam to carbon ratio of 1.0, giving little coke on the catalyst(<1 wt.-%), whereas the commercial steam reforming catalyst lost its activity at 20 h because of severe coking under the same reaction conditions. For both Ni0.03Mg0.97O and Ni/Al2O3-MgO, the rate equations were similar. Thus it was suggested that the suppressed coke formation of our catalyst is caused by the very small nickel particles and the difference in the composition of surface carbonaceous species on the Ni metal particles as the reaction intermediate.