A multi-party session key agreement method includes: a test session for exchanging a short-term key between parties of 3 to n peers; and an original session for exchanging a long-term key between the parties who have exchanged the short-term key. Peer (n) that has conducted the test session and the original session has cluster (n) that manages the keys as a result of conducting the sessions, and cluster (n) agrees with a result of the session conducted in peer (n) by communicating with cluster (n+1) of another peer (n+1).