A content distribution system encrypts a content by using different session keys assigned to user systems, encrypts each of the session keys with a public key corresponding to a decryption key unique to each user system, generates, for a group of user identification information items, header information including the encrypted session keys, and a first vector which corresponds to a session key of the session keys and is assigned to arbitrary user identification information u in the group, the first vector being set such that an inner product of the first vector and a second vector concerning the user identification information u becomes equal to zjuv (where zj is a constant value of a session key sj assigned to the user identification information u, and v is group identification information to the group), and transmits the header information and one of the encrypted contents to the user systems.