PURPOSE: To connect a wired OR signal line without interlocking in a bus connection device connected to the bus of an extension box or the other system through a signal transmission line whose signal form is different from the bus.

CONSTITUTION: Substitution mechanisms 46 substituting the level of the wired OR signal line 51 of the other bus are provided for respective systems. The level of the signal line 51 is transmitted to the substitution mechanism 46 of the other system through a channel. When the substitution mechanism 46 is in a prescribed level, the system prevents the level of the signal line 51 from being transmitted to the other system. Thus, it is avoided that the transmission of the level avoids the continuation of echoing between the systems, and interlocking is released. Re-substitution mechanisms 47 making the mirror images of the substitution mechanisms of the other systems are provided for both systems. A prescribed level is transmitted to the substitution mechanism 47 of the other system based on the non-matching of the level of the re-substitution mechanism 47 with the level of the signal line 51 of the system.