Owing to rapid expansion of the Japanese economy, the object of public investment is now changing from that of ‘stagnation’ economics to ‘ bottle-neck’. The relative advantage of unbalanced growth of private investment to public has been gradually diminished and inefficiency due to the unbalanced growth has manifested itself and become the fetter to the economic growth in private sector. Therefore the determination of amount of investment (need) in public sector which has been and is less developed, and the optimum investment share to the private (consistency) are the primal target of economic policy. Here we are concerned with the transport investment, above all, road investment, and try to estimate empirically the requisite amount of investment corresponding to national economic activity level, and its consistency among other several public investment, considering the actural data in the past.