An MFP can transmit image data read by a scanner by attaching it to E-mail. At the time of transmission, the image data input by the scanner and stored in a work memory is converted into mail data so as to be transmitted by E-mail, and the mail data is stored again into the work memory. The mail data is divided every predetermined reference data size into divided files which are sequentially transmitted as divided mails. When completion of transmission of divided mails corresponding to a unit image is confirmed, the image data and mail data of the unit image are eliminated from the work memory. By sequentially performing such a process, the memory use efficiency can be improved while holding image data of a unit image which has not been transmitted yet.