A FAX document image is acquired from an external device having a FAX receiving function, a telephone number of a transmission source of the FAX document image is acquired, a folder of a document management system as a destination of registration is automatically determined based on a mapping table showing a mapping between the telephone number of the transmission source and the registration destination folder, and the acquired telephone number of the transmission source, the FAX document image is registered in the folder of the document management system as the destination of registration in a way that attaches an index to the FAX document image.