PURPOSE: To reduce the man-hour required for expansion work and to improve the economy by inserting and coupling a through-package providing an expansion cable prepared in advance to a connector from which a package of a terminating device is being extracted and connecting the other end of the cable to a terminating device fitted to a new processor.

CONSTITUTION: In order to prolong a communication bus cable and to connect a new processor CPUn+1, terminating packages TRO, TRI are removed from a processor CPUn, through-packages THO, THI for communication bus signal- through are mounted instead, and the remotest end of bus cable is connected to the front connector. Then, cables CBOA, CBIA formed newly are connected to the other end of the front connector of the through-packages THO, THI and connected to the processor CPUn+1, then the removed terminating package is mounted and the expansion work is finished by connecting the cables CBOA, CBIA thereto.