An encryption processing part of a controller of each of user devices specifies components of content as encryption target parts, creates an encryption key creation key by reading out a public key, for each of the encryption target parts, of a second user who is allowed to view the encryption target part, raising a secret key of an administrator of the content data to the power of the public key thus read out, and calculating a remainder, encrypts random numbers each corresponding to each of the encryption target parts by using the encryption key creation key, creates an encryption key for each of the encryption target parts, and encrypts, by using the encryption key created for each of the encryption target parts, each of the encryption target parts corresponding to the encryption key.