PURPOSE: To reduce the transfer time of data by sending a data to a line while being split into a segment size corresponding to the least data transfer quantity in the data speed through the line in adding/disconnecting lines so as to improve the utilizing efficiency of the line.

CONSTITUTION: In adopting multi-link configuration of lines 01∼04 to accommodate the data transfer equantity as shown in figure, a split size revision section 4 reads a data '300B/S' being the least data transfer equantity from a split size check table 5 and the value is stored in the mode to be revised into the split size 3. Thus, a segment split section 2 splits the transfer data (BTU) transferred from the host into segments corresponding to the said split size 3. The split segments are sent to the lines 01∼04 as a line transmission data (piu). Thus, the utilizing efficiency is improved by preventing an idle time for the lines in use to shorten the data transfer time.