A microcomputer input/output circuit is provided with a microcomputer, a power supply, a power supply resistor that is connected at one end to the power supply and alternately connected at an another end to grounded external resistors via a switch, a power supply voltage monitoring unit capable of monitoring a power supply voltage, a divided voltage value monitoring unit capable of monitoring a divided voltage value obtained through voltage division by the power supply resistor and a selected one of the external resistors and an external resistor specification means that specifies the external resistor selected by the switch depending on which divided voltage value range corresponding to each of the external resistors the divided voltage value is included, and the external resistor specification means A changes the divided voltage value ranges based on the power supply voltage acquired by the power supply voltage monitoring unit.