Decentralized and unstructured peer-to-peer (P2P) networks such as Gnutella are attractive for Internet-scale information retrieval and search systems because they require neither any centralized directory nor any centralized management of overlay network topology and data placement. However, due to this decentralized architecture, current P2P keyword search systems lack useful global knowledge such as popularity of data items and relationships between keywords and data items. As a result, current P2P keyword search systems supports only naive text-match search and can find only data items with a keyword (or meta-data) exactly indicated in a query. In this paper, we show an efficient P2P search system which increases possibility of discovering desired data items. The key mechanism is query expansion, where a received query is expanded based on keyword relationships managed in a distributed fashion by participating nodes. Keyword relationships are improved through search and retrieval processes and each relationships is shared among nodes holding similar data items. We also present implementation of our P2P search system.