To attain live line pull in and out for a terminal without giving effect on communication during the communication between optional terminals by applying live line pull in/out to the terminal being an object of live line pull in and out in a state that an interruption means is started to interrupt a corresponding signal line during the communication between the optional terminals.

In the case that an operation is made to connect an optional terminal 2k to a multi-terminal bus 1 or to disconnect an optional terminal 2k1 already connected from the multi-terminal bus 1 when terminals, e.g. 2a, 2b make communication with each other on the multi-terminal bus 1, each of bus interruption sections 3k-3k3 interrupts temporarily buses 4k-4k3 between the multi-terminal bus 1 and the terminals 2k-2k3. That is, the terminals 2k-2k3 are disconnected from the multi-terminal bus 1 by using the bus interruption sections 3k-3k3. Then the disconnected state is recovered as it was by the bus interruption sections 3k-3k3.