This transmission input circuit is provided with an adjustment processing section which turns ON a switch at an empty timing where transmission current from a slave device is not flowing, to allow a reference current to flow from a constant current circuit to a current detection resistor, generates in the current detection resistor a target adjustment voltage, in which a threshold voltage corresponding to the reference current is added to a load current detection voltage corresponding to the load current, and adjusts a digital value so that a reference voltage output from a digital variable resistor matches with the target adjustment voltage.