A reformer using a circulating fluidized bed has been proposed for the production of hydrogen and syngas. A catalyst is circulated between two reactors for the catalytic decomposition of methane to hydrogen and carbon, and for the gasification of carbon deposited on the catalyst. Catalytic activities of Ni/SiO2 for both CH4 decomposition and CO2 gasification of deposited carbon were investigated at 873-1023 K in a periodically-operated fixed bed reactor. Carbon which had a moss-like morphology was hardly gasified and caused deactivation of the catalyst. Such a type of carbon became dominant with raising reaction temperature and with increasing amount of Ni loaded on SiO2. In contrast, filamentous carbon which had a graphitic structure could be gasified with CO2 and became dominant with decreasing amount of Ni loaded and with lowering reaction temperature. It was found that Ni(5 wt%)/SiO2 had a stable activity against the cycles of decomposition/gasification because Ni on Ni(5)/SiO2 was difficult to sinter at 873 K and as a result, the filamentous carbon was exclusively formed.