A user apparatus cannot acquire as many distribution keys Ki, . . . as a threshold value, to restore a content encryption key KM, unless it decrypts as many encrypted sub-content items as the threshold value after it has played back a sub-content item SC1. The user apparatus monitors the state in which the sub-content is being played back. It outputs a warning when the sub-content is in fast-forwarding state or skipping state. If the playback state is not returned to one before the fast forwarding or skipping, in spite of the warning, the user apparatus will erase the distribution keys acquired in the fast-forwarding state or skipping state.