An input module of an industrial control apparatus receives a digital signal from an external source, outputs a binary signal as an internal signal, and determines a level of the digital signal based on the internal signal. The input module determines that the digital signal has a low level when a low level of the internal signal continues for a certain time period, determines that the digital signal has a high level when the internal signal switches from the low level to a high level and then switches to the low level again, and determines an occurrence of a fixing failure when the internal signal switches from the low level to the high level and then fails to switch to the low level again.