A power supply control part supplies offset voltages from power supplies to each of signal wires at start time and then, after a predetermined delay time period, supplies a reference voltage from a power supply. A series resistor circuit of resistor elements is provided in each signal wire and a resistor element is connected between a supply node of the offset voltage and an input terminal of the series resistor circuit. A resistor element is connected between the supply node of reference voltage and the output terminal of the series resistor circuit provided in the signal wire of a high-potential side. A comparison logic part detects a disconnection of the signal wire based on a voltage developed at the input terminal of itself during the delay time.