Since methane decomposition has no CO2 emissions, it is attracting attention as a hydrogen production method with a low environmental burden. Understanding the structure of the produced carbon is important for longterm stable production of hydrogen. In this study, methane decomposition was carried out on carbons with several different structures (activated carbon (AC), carbon black (CB), meso-porous carbon (MC), and carbon nanofiber (CNF)). We have found that the carbon produced by methane decomposition decreases activity by covering the catalyst, but itself also acts as a catalyst irrespective of the original carbon catalysts. All of the catalysts continued to maintain a methane conversion ratio of about 17% by catalyzing the produced carbon even after the activity was lowered. By analysis of the catalysts before and after the experiment, it was shown that the produced carbon covered the catalyst surface and resulted in a specific surface area of about 10 m(2)/g and the intensity ratio of the D band to the G band in the Raman spectra (I-D /I-G) of around 1.55 irrespective of the original carbons structures. We proposed that Raman spectroscopy is an effective method for evaluating initial catalytic activity for methane decomposition because I-D/I-G of the catalysts before the experiment have a linear relationship with the methane conversion ratio per unit surface area in the early stage of the reaction.