An apparatus for detecting noise in a power supply of a PLC analog input/output module is provided. The apparatus may include a capacitor configured to extract a noise determination target voltage from a driving voltage outputted from the power supply; a voltage conversion unit configured to convert the noise determination target voltage outputted from the capacitor and to output the converted noise determination target voltage; a comparison unit configured to compare the noise determination target voltage converted by the voltage conversion unit with a reference voltage; and a determination unit configured to determine whether the noise determination target voltage is a noise, based on a comparison result by the comparison unit.