The effects of the reformer in an internal-reforming molten carbonate fuel cell (IR-MCFC) are studied by mathematical modeling. Temperature distributions, conversion of methane and compositions of gases are analyzed through mathematical modeling of the reformer and the cell. In the reformer, the methane-reforming reaction and the water-gas shift reaction occur simultaneously and the conversion of methane to hydrogen, calculated including the thermodynamic equilibrium of the reaction, reaches 99%. Additionally, the endothermic-reforming reaction contributes to a uniform temperature distribution. The voltage and the power of the IR-MCFC are similar to those of an external-reforming molten carbonate fuel cell (ER-MCFC), when the compositions at the inlet of the ER-MCFC are set as those at the outlet of the reformer in IR-MCFC. As the molar ratio of methane to water-gas decreases at a fixed total flow rate, the working voltage decreases.