To approve and authenticate a document with an approval image and prevent the approval image from being stolen.

At a document approval request (101), a document ID is generated (102), the approving person is authenticated (103), and a ciphered document ID is generated from the document ID and approving person ID (104); and a modified approval image is generated by modifying the approval image data with the ciphered document ID (105), and the document ID approving person ID and modified approval image are inserted into the document (106). At a document authentication request (111), the document ID, approving person ID, and modified approval image are extracted from the document (112), and the ciphered document ID is extracted from the modified approval image and an inputted approval image (113); and the restored document ID is decoded by using the ciphered document ID and approving person ID (114), and when the document ID and restored document ID match each other, authentication OK decision is made (116).