Data protection system that protects data by encrypting the data

A decryption key determining device that determines decryption key groups for use in decryption to be individually assigned to at least three terminals that obtain encrypted data and decrypt the obtained encrypted data, comprising a decryption key setting unit for associating each terminal with a leaf in a tree structure, and determining, for each node in the tree structure, decryption keys in correspondence with an invalidation state that indicates whether or not the terminal corresponding to a leaf that is reachable from the node is invalidated; and a decryption key group assignment unit for determining, for each terminal, a decryption key group to be assigned to the terminal, based on the decryption keys determined for each node that is on a path from the leaf corresponding to each terminal to a root in the tree structure.