An image processing apparatus of this invention converts image data into a predetermined transmitting format in turn from a read page before images are read from all pages of originals. Upon completion of reading operations of all the pages of the originals, the image processing apparatus controls a transmitting unit to transmit image data in association with the image data of a page, which has already been converted into the transmitting format, and controls to convert image data into the transmitting format and then controls the transmitting unit to transmit the image data, in association with the image data of a page, which has not been converted into the transmitting format.