The image data transmitting apparatus acquires the address book data corresponding to the user or such data and the transmittal cover letter data from the server apparatus, selects, out of the acquired information on destinations, the destination according to selecting operation by the user, generates transmittal cover letter image data based on the transmittal cover letter data related to the selected destination and attaches the image data to be transmitted as second and subsequent pages, thereby generating the image data with the transmittal cover letter attached thereto. While thus generated image data with the transmittal cover letter attached thereto is transmitted to the destination, transmission is prohibited of image data other than thus generated image data with the transmittal cover letter attached thereto.