An ID creating apparatus includes a derivative value creating unit for creating a derivative value, a user ID encrypting unit for creating encrypted ID by encrypting information including the user ID and the derivative value based on an encryption key, and a derivative ID creating unit for creating derivative ID based on the encrypted ID. An ID resolving apparatus includes an encrypted ID extracting unit for extracting the encrypted ID included in the derivative ID, an encrypted ID decrypting unit for decrypting the encrypted ID based on a decryption key corresponding to the encryption key and thereby creating decrypted ID, and an ID judging unit for determining that the derivative ID is created from the user ID when part of the decrypted ID coincides with information created based on the user ID.