An apparatus is provided to determine deterioration of a photocoupler. The apparatus includes a detecting unit and a determining unit. The detecting unit receives an electric pulse signal outputted from the photocoupler. The amplitude of the outputted pulse signal depends on that of an electric AC voltage applied to the photocoupler. The detecting unit detects a duty ratio of the pulse signal. The determining unit determines whether or not the duty ratio is less than a threshold given for the determination. The determining unit also determines that the photocoupler has deteriorated in performance thereof more than a usable level if it is determined that the duty ratio is less than the threshold given for the determination.