PURPOSE: To improve the transmission efficiency by reducing the quantity of data to be sent again on the occurrence of a transmission error.

CONSTITUTION: Suppose that a bit error rakes place on a physical line 5, the re-transmission request is made in compliance with the international recommendations X.25, since the data is sent again in the unit of frames, only the data shared to each of SLP control sections 3a-3d are sent. Thus, it is enough to send data again being nearly 1/4 in comparison with re-transmission of data in the unit of frames inputted in an MLP control section 2 in a conventional system.