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 degreesC) 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 hotwater 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 g-as are considered. As a result, the optimum scale of a co-generation system a with DIR-MCFC and using city gas is 3 kW, while it is 6 kW 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.