We proposed and analyzed the win-win relationship between large-sized and small-sized companies. More specially, the concept of relational investment is proposed as a norm of win-win relationship management between large-sized and small-sized companies. In short, a certain amount of efforts and investments are necessary to build the win-win relationship between large-sized and small-sized, what we call the relationship-specific investment. These include investments for mutual understanding, investments for mutual adjustment and support, and investments for joint work. Without these investments it is also hard to expect to construct the win-win relationship. We analyzed six cases of relationships between large-sized and small-sized companies to verify the role of relationship-specific investments. The results show that various relational investments were realized across all six cases. The finding implies that the relational investment can be a key factor of managing the win-win relationships between large-sized and small-sized companies.