According to one embodiment, a method for authenticating a device, wherein the device holds secret identification information, encrypted secret identification information, and key management information, and an authenticator holds an identification key, the method includes reading, by the authenticator, the encrypted secret identification information and the key management information from the device, and obtaining, by the authenticator, a family key by using the key management information, the family key being capable of being decrypted with the identification key. The method further includes obtaining, by the authenticator, the secret identification information by decrypting the encrypted secret identification information with the family key.