Nowadays, dramatic improvement of interregional transports system, particularly high-speed train systems, are booming in Europe and other regions of the world. Naturally their impacts on restructuring of the regional economy are most interested. Interregional input/output model is one of the tools to analyze such a phenomenon, however, it cannot provide actual trade amount between the regions, because it is based on the deterministic theory. In this study, using random utility theory, we first develop a theoretical basis of the combined model of interregional imput/output model and spatial interaction model. So that we get the following three equations as the equilibrium conditions: productivity equation, price equation and coefficient equation of trade. Under the constraint that the solutions equal to the above equilibrium conditions, we can also specify a programing model, which provides a social welfare function. Then the Chenery-Moses model and Amano-Fujita model can be interpretted as a special case of our model. And Moses model and Wilson model in the form of entropy specification are interpretted as a simplified version of our programing model.