It is very important to manage digital documents and documents obtained by printing the digital documents. After completion of a conference, each participant makes an MFP read a document image of a distributed reference. The MFP searches for a data file which matches the read document image with reference to information in a storage unit, and deletes the detected data file. If the number of pages of the read image is short, the data file cannot be detected, or the read document image shows a partial loss, the MFP generates a warning. Upon reception of a collection completion message, the MFP checks with reference to information in the storage unit if all data files associated with the conference are deleted. If a data file to be deleted still remains and that data file is printed, the MFP generates a warning.