PURPOSE: To prevent a delay in a response from a center equipment from being caused by preventing a communication interruption from other terminal equipment to a terminal equipment during data transmission with respect to the data transmission control system in which plural terminal equipments connected with a multidrop line to a center equipment make data transmission to the center equipment in a contention form.

CONSTITUTION: Each terminal equipment 1 is provided with a carrier detection section 18 detecting an idle communication line and outputting an off-signal and with a CD holding circuit 17 outputting a CD signal set for a prescribed time even after an output of the carrier detection section 18 is OFF. Moreover, a holding time of the CD latch circuit 17 is controlled to be reduced during data transmission from its own terminal equipment and the terminal equipment making data transmission detects earlier an idle communication line than the other terminal equipment to implement transmission of a succeeding data block with priority over the other terminal equipment.