A telephone directory information modifying apparatus which makes use of backup data, which is a backup of telephone directory information of portable telephones and held at a data center, to permit mutual operation accompanying telephone number change. On receiving a telephone number change notification instruction, changer telephone directory information retrieving means retrieves telephone directory information of a changer from the backup data, registrant information extracting means extracts information about registrants registered in the telephone directory, registrant telephone directory information retrieving means acquires registrants' telephone directory information from the backup data, telephone number change recipient extracting means extracts registrants who have registered the changer in their telephone directory information, telephone number changing means rewrites the changer's telephone number in the extracted registrants' telephone directory information, and change notifying means transmits telephone number change notification mail to the registrants.