Upon receipt of a service data request from a client, a license server offering a key to decrypt a content transmits a user information request to the client. On receiving the user information request, the client displays a message prompting its user to enter such user information as the user's personal information and accounting information. The user information thus entered is sent from the client to the license server. On receiving the user information, the license server assigns a leaf of a key-managed hierarchical tree structure to the client, generates a set of node keys as a device node key, sends to the client the device node key together with a leaf ID and a private key of the client in question, and records the user information in correspondence with the leaf ID.