A data transmission system includes a transmitter, a receiver and a transmission line. The transmitter produces data on the transmission line for a predetermined time, and the receiver reads in repeatedly and consecutively the data during the predetermined time. A predetermined number is set for repeatedly reading-in the data by the receiver. The number is determined by an external condition during a period of time in which the data are produced. The receiver accepts the data only when the data is read without any change for the determined number of times.