PURPOSE: To prevent malfunction from occurring even when a terminal is connected to a signal line with reversed polarity by judging connection polarity to the signal line from the polarity of the start pulse of a transmission signal, and conforming the polarity of the start pulse to that regulated in advance.

CONSTITUTION: It is judged that with what polarity its own is connected to the signal line (l) by a transmission control circuit 2 based on from which comparators CP1, CP2 output can be obtained when the start pulse of a start-up packet is received, and the circuit 2 transmits the transmission signal so that the start pulse can be set as a pulse at a (+) side corresponding to the connection polarity. Therefore, the terminal 1 can be connected to the signal line (l) with non-polar, and the transmission signal in which the polarity of the start pulse is changed to the one with regulated polarity can be transmitted by the terminal 1, thereby, no malfunction occurs when the terminal 1 is connected to the signal line (l) with the reversed polarity.