A digital signature apparatus including, a converting unit that converts, based on a first video image frame being independently replayable, a predicted frame being not independently replayable into a second video image frame being independently replayable, an encoding unit that encodes the first or second video image frame into an image data according to an image format, a transfer unit that transfers, when receiving the predicted frame, the predicted frame to the converting unit, and transfers, when receiving the first or second video image frame, the received video image frame to the encoding unit, and a digest information generating unit that generates a digest information for each of image data encoded by the encoding unit.