Each of a plurality of contents supply systems connected to a single user apparatus uses a group management method that employs flexible and unique tree structures. A manager in each contents supply system uses a system apparatus key distributed by a key management organization to generate a public list that corresponds to a unique tree structure, and releases the public list when the contents supply system is constructed. The user apparatus stores only an apparatus key corresponding to a leaf of the tree structure. Using the public list, which is released via a web page, package media or the like, the user apparatus reconstructs the tree sequentially from the leaf up, and derives a node key corresponding to the user apparatus. The user apparatus then decrypts an encrypted content using the derived node key.