An authentication device, an authentication method and an authentication program of the present invention reduce a load occurred when registering digest information of a file in an authentication server and a load occurred when demanding authentication, and thus facilitates the authentication. Address information of a registration object file is received, the file on the basis of the address information is acquired, timestamp information of the file is acquired, digest information of the file acquired by a file acquisition module is generated, the digest information and the timestamp information are registered in a file registration module in a way that associates the digest information and the timestamp information with each other, the file is authenticated in response to a demand for authentication by comparing the digest information of the authentication object file with the digest information registered in said file information registration module, and, when authenticated by said authentication module, the timestamp information of the file is outputted.