PURPOSE: To prevent a fault from occurring in an assembled line adaptor after a hot line is inserted and a line is re-assembled in a system comprised of a line control common part and the line adaptor connected to a bus and to/from which the hot line can be inserted/withdrawn.

CONSTITUTION: The line adaptors 20-2n are provided with reset circuits 32 which initialize the line adpator after the hot line is inserted, and effective flags 33 representing the effectiveness of the line adaptor. Also, a line control common part 10 is equipped with a microprogram which runs the test programs of the line adaptors 20-2n. After the line adaptors 20-2n are inserted, the initialization of the line adaptors 20-2n are executed, and the effective flags 33 are read out by the microprogram of the line control common part 10, then, the test program is executed on the line adaptor for which the initialization is completed.