A communication system that can quickly perform the processing of the data re-transmission when error occurs in the data. The communication system includes an automatic original feeder and an image reader that transmit and receive data simultaneously. A CPU provided in the automatic original feeder determines error of reception data. The CPU adds the result of the determination to transmission data being transmitted to the image reader. One of the automatic original feeder and the image reader acts as a slave, and the other acts as a master.