A safety input device includes an input controller to control transmission of an input signal to an arithmetic device, an output controller to instruct output of preset first self-diagnosis pattern data, a test pattern generating unit to encode the first self-diagnosis pattern data into a pulse train signal having a pulse width equal to or below a predetermined value and output the pulse train signal, a combination input unit to combine the pulse train signal with the input signal, an input interface unit, a pattern reconfiguration unit to decode an output signal from the combination input unit into second self-diagnosis pattern data, and a comparator to compare the first and second self-diagnosis pattern data to judge the presence or absence of a difference between the first and second self-diagnosis pattern data.