The decomposition of CH4 into C and H-2 over Ni/SiO2 was investigated with a Pd-Ag hydrogen-permeating membrane reactor. The CH4 decomposition reaction hardly proceeded at temperatures lower than 773 K in a conventional fixed-bed reactor because of a chemical equilibrium limit. However, the conversion of CH4 was dramatically increased by removing the formed hydrogen by the Pd-Ag membrane. Because higher CH4 decomposition conversions were achieved at higher hydrogen removal rates in the Pd-Ag membrane, the removal of the formed hydrogen seems to be the key step for CH4 decomposition reaction at decreased temperature. Because a higher hydrogen permeation rate was achieved on the Pd(77)-Ag(23) membrane than on the Pd(90)-Ag(10) membrane, the CH4 conversion was always higher in membrane reactors using Pd(77)-Ag(23) than in reactors using Pd(90)-Ag(10) for the hydrogen separation membrane. The CH4 conversion increased with increasing contact time of the reactant and/or with increasing sweep Ar flow rate, because the hydrogen removal rate was improved. Consequently, this study revealed that CH4 decomposition into CO and H-2 can occur with sufficiently high conversion (>88%) at 773 K and this process can provide hydrogen at decreased temperature.