PURPOSE: To enable continuous data transmission without waiting for reply as to vacancy of the buffer from the receiving side by sending out data continuously as long as there is the status display element in reset state.

CONSTITUTION: In the receiving side transmitter 13, control information is sent out to the transmitting side when areas A, B, C, D of a buffer memory 7 become empty. In the transmitting side transmitter 1, one of status display elements 16 is reset every time when the control information is received. The transmitting side sends out data continuously as long as there are status display elements 16 in reset state. Thus, data transmission can be made continuously without waiting for reply from the receiving side 13 as to whether the buffer 7 is vacant or not.