A digital work protection system composed of (a) user apparatuses that are recording apparatuses and/or reproduction apparatuses for recording or reproducing digital contents such as movies, (b) a recording medium, and (c) a key management apparatus that manages the assignment of device keys to the user apparatuses using a tree structure. The key management apparatus always keeps some leaves with which no user apparatus is not correlated in the tree structure, generates new leaves that are connected from the leaves and belong to a new layer, and assigns new user apparatuses to the newly generated leaves.
