PURPOSE: To obtain a transmitter at a low cost in which whether or not a power supply voltage is superimposed on an information transmission line is detected and whether or not a connection terminal of a terminal equipment is connected to the information transmission line in a positive or a reverse polarity is discriminated.

CONSTITUTION: A transmission control circuit 6 establish that other terminal equipment superimposes a power supply voltage on an information transmission line when a current flows through a light emitting diode of a photocoupler 20 while a switch 24 is turned off and also positive pole connection of connection terminals 17, 18. Moreover, the transmission control circuit 26 establishes that other terminal equipment superimposes a power supply voltage on the information transmission line when a protection circuit 21 outputs an overcurrent detection signal while the switch 24 is turned for a prescribed period of time and also reverse pole connection of the connection terminals 17, 18. The transmission control circuit 6 establishes that other terminal equipment superimposes a power supply voltage on the information transmission line when no current flows through a light emitting diode of the photocoupler 20 and no overcurrent detection signal is outputted while a switch 24 is turned on.