The possibility of introducing a co-generation system with a direct internal reforming-molten carbonate fuel cell (DIR-MCFC) for residential use is examined by a feasibility study. First, the structure of a system, which can maintain the cell temperature (650°C) without the heat supply, is constructed by calculating heat and material balances among the system components. Secondly, a model family, which might use the co-generation system with a DIR-MCFC, is constructed from the results of a questionnaire on room layout, number of family members, and the number of electric appliances and consumption of electric power in Osaka. Thirdly, calculating the electric power and hot-water demand supply balance optimizes the scale of the co-generation system with a DIR-MCFC for residential use. Finally, the running costs of this optimum system using city gas or propane gas are considered. As a result, the optimum scale of a co-generation system a with DIR-MCFC and using city gas is 3kW, while it is 6kW for the case using propane gas. The co-generation system using city gas is suitable for a house. On the other hand, the system using propane gas is suitable for an apartment.