A transmission input circuit of a master device which, in a state where a load current from a load is flowing into a transmission line serving also as a power supply line, detects the presence or absence of a transmission current from a slave device, the transmission input circuit comprising: a current detection resistor which receives input of a line current flowing through the transmission line and generates a line current detection voltage, characterized in that the transmission input circuit further comprises: a switch which, by performing switching, allows a predetermined reference current from a constant current circuit to flow to the current detection resistor and generate a threshold voltage; one of a digital variable resistor circuit which sets a resistance value according to a digital value to thereby generate an arbitrary reference voltage, and a DA converter which generates the arbitrary reference voltage according to a digital value; a comparator which receives an input of the line current detection voltage generated by the current detection resistor, to one input terminal, receives an input of the reference voltage generated by the one of the digital variable resistor circuit and the DA converter to another input terminal, and outputs a component of the line current detection voltage which exceeds the reference voltage, as a transmission current detection signal; and an adjustment processing section which turns ON the switch at an empty timing where the transmission current from the slave device is not flowing, to allow the reference current to flow from the constant current circuit to the 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 the digital value so that the reference voltage output from the one of the digital variable resistor circuit and the DA converter matches with the target adjustment voltage. a current detection resistor which receives input of a line current flowing through the transmission line and generates a line current detection voltage, characterized in that the transmission input circuit further comprises: a switch which, by performing switching, allows a predetermined reference current from a constant current circuit to flow to the current detection resistor and generate a threshold voltage; one of a digital variable resistor circuit which sets a resistance value according to a digital value to thereby generate an arbitrary reference voltage, and a DA converter which generates the arbitrary reference voltage according to a digital value; a comparator which receives an input of the line current detection voltage generated by the current detection resistor, to one input terminal, receives an input of the reference voltage generated by the one of the digital variable resistor circuit and the DA converter to another input terminal, and outputs a component of the line current detection voltage which exceeds the reference voltage, as a transmission current detection signal; and an adjustment processing section which turns ON the switch at an empty timing where the transmission current from the slave device is not flowing, to allow the reference current to flow from the constant current circuit to the 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 the digital value so that the reference voltage output from the one of the digital variable resistor circuit and the DA converter matches with the target adjustment voltage. a switch which, by performing switching, allows a predetermined reference current from a constant current circuit to flow to the current detection resistor and generate a threshold voltage; one of a digital variable resistor circuit which sets a resistance value according to a digital value to thereby generate an arbitrary reference voltage, and a DA converter which generates the arbitrary reference voltage according to a digital value; a comparator which receives an input of the line current detection voltage generated by the current detection resistor, to one input terminal, receives an input of the reference voltage generated by the one of the digital variable resistor circuit and the DA converter to another input terminal, and outputs a component of the line current detection voltage which exceeds the reference voltage, as a transmission current detection signal; and an adjustment processing section which turns ON the switch at an empty timing where the transmission current from the slave device is not flowing, to allow the reference current to flow from the constant current circuit to the 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 the digital value so that the reference voltage output from the one of the digital variable resistor circuit and the DA converter matches with the target adjustment voltage.