This article analyzes the structure of the hyperlink network formed by the Web pages of Japanese public libraries and its relationship with the network formed by Inter-Library Loans (ILLs), the traditional system of cooperation among public libraries. Our results indicate that (a) the hyperlink network is effectively connected in the sense that each library is reachable from other libraries by clicking a few links and (b) the network has many groups of libraries that cooperate with each other. Most of the cliques consist of prefectural libraries or libraries in the same prefecture. The hyperlink network shows two of the same tendencies as the ILL network: (a) The connection among libraries that are geographically close to each other or in the same administrative unit is strong, and (b) prefectural libraries occupy the central position. There are differences between them, however, in terms of the amount of ILLs and cliques. We conclude that Japanese public libraries have formed a network on the Web that is strongly affected by traditional cooperation, but also incorporates some new types of cooperation from the perspective of cliques.