PURPOSE: To eliminate interference of communication with other apparatus by detecting lowering of power source voltage level and separating the driver from signal line and thereby not outputting unnecessary signal to the common signal line that connects plural apparatus.

CONSTITUTION: When supply power source voltage VCC of terminal unit connected to a bus line lowers, the voltage VCC is detected by a voltage level detector 11 and a control transistor Q3 is made conducting. Accordingly, an output transistor Q1 becomes nonconducting, and the terminal unit is disconnected electrically from the bus line and does not disturb communication of other terminal units. In the case where the level of power source voltage VCC lowers and transistors Q2, Q3 in the detector 311 do not perform controlling action, the output transistor Q1 comes to off state, and consequently does not disturb communication of other terminal stations.