Provided is an image data transmitter in which, in the event of transmission of the same image data to multiple recipients, the image data with added supplementary information different for each of the multiple recipients can be transmitted through easy operation. Further provided is an image data communication system using the image data transmitter. An image of a document is read out, and a destination of the image is set. Multiple destinations can be set. Then, selection is performed whether or not to input supplementary information. When performing input processing, a destination to which the supplementary information is to be transmitted is selected from the multiple destinations. Subsequently, the supplementary information for the destination is input, and the supplementary information is correlated with the destination. Additionally, the image data and the supplementary information are combined. Then, composite data for the destination is transmitted to the destination.