This article is an introduction and comment for the "An Econometric Model of the United States by L. R. Klein and S. Goldberger. They have analysed for economic system with simultaneous equations and tried to predict a large number of economic variables by statistical method. According to their persistence, the severest test of any theory is that of its ability to predict. But, in spite of this assertion, the parameters of their equations failed to support their stability. Moreover, in case they have increased the experimental period only two years, some equations changed their formulation. In the long run, the stability of parameter can not be supported by economic theory. We should not to assert its stability, but try to find the principle to explain their change-e. g. the change of industrial structure. This method is to be completed by the theory of capital accumulation.