An input/output apparatus for a PLC includes at least first and second sub-systems. In each sub-system, a DC-DC converter controls a voltage of an external power supply to a target voltage by switching a PWM signal, and a microcomputer is driven by a clock signal. In the microcomputer, a PWM signal generating unit generates the PWM signal and a frequency analyzing unit samples an inputted voltage at a cycle shorter than a cycle of the PWM signal, and analyzes the frequency of the inputted voltage. The frequency analyzing unit acquires a frequency of the PWM signal from the frequency of the inputted voltage, calculates a frequency of the clock signal, and outputs an abnormality-notifying signal when the frequency of the clock signal falls outside a prescribed range. An output voltage of the DC-DC converter of the first sub-system is inputted to the frequency analyzing unit of the second sub-system.