An image forming apparatus having a network communication function, includes a data input unit which inputs digital data; a data transmission unit which transmits the input digital data to a data management server for management of data; a digest generation unit which generates a digest of the input digital data; a digest storage unit which stores the digest generated by the digest generation unit; a time stamp acquisition unit which acquires a time stamp for authentication of an input time of the digital data from a time stamp station by issuing a time stamp request including the stored digest at a scheduled time; and a data management directive unit which transmits the acquired time stamp to the data management server for management of the time stamp and the transmitted digital data, associated with each other.