We propose a semi-structured peer-to-peer (P2P) network for file sharing built from a structured P2P network as a basis for file search together with an unstructured P2P network for access load balancing. This semi-structured P2P network adopts a structured P2P network capable of providing reliable file search as its base structure. In addition, it makes lightly loaded nodes in the structured network construct unstructured P2P networks over which heavily loaded nodes can spread their access load. The semi-structured P2P network is required to dynamically reconstruct its network topology according to changes in node loadings and file search performance. We also propose an autonomic, distributed, and cooperative method for dynamically altering the network topology. In simulation experiments, we first obtain a network topology that ensures reliable file search and is optimal in terms of access load balancing. This is carried out under the (idealized) assumption that the load state of all nodes in the network can be obtained in advance. We also show that the proposed topology reconstruction method yields network topologies with almost the same performance as the optimal configuration.