The invention provides an image data transmitting apparatus and functions which eliminate needs of troublesome handling of documents without reducing utilization efficiency of the apparatus in retransmitting document data. The image data transmitting apparatus comprises a document reading portion; an image memory; a communication portion; and a main CPU, in such a configuration that when a transmission trouble is detected, the apparatus stops both reading operation of documents and transmission operation of image data and, when the documents are reset thereafter, resumes both of the reading and transmission operations, in which the apparatus stores both the number of document sheets for which image data has been transmitted by a time when transmission is stopped due to a transmission trouble and the number of documents for which image data has already been stored. In retransmitting all the documents, the apparatus provides discrimination data for discriminating between documents for which image data is not transmitted and documents for which image data is already transmitted, based on the stored number of transmitted sheets.