[en] In recent years, considerable attention has been paid to carbon dioxide reforming of natural gas, namely CO₂ reforming, since it can produce synthesis gas with low hydrogen-to-carbon ratio preferentially used for production of liquid hydrocarbons in the Fischer-Tropsch and methanol syntheses. This reaction has also very important environmental implications because CO₂, a green house gas, may be converted into valuable feedstock. In JAERI, CO₂ reforming using the out-of-pile test facility, which is a 1/30 scale model of the HTTR hydrogen production system, is also being considered as an application of steam reforming. For the purpose to estimate the reformer performance in the facility, numerical analysis of natural gas reforming processes of CO₂ and combined reactions with steam and CO₂ has been carried out using mathematical model on heat and mass balance accompanied by chemical reactions. The reformer performance was evaluated in the effect of pressure, temperature, process gas composition and reaction rate constants of the catalyst on conversion, product gas composition and heat consumption of He gas. And also, the potential of carbon formation by CH₄ cracking reaction and Boudouard reaction was estimated. (author)