A safety device includes a first computation unit, a second computation unit, an output control unit, and a first central processing device. The first computation unit is configured to perform a first computation on a detected value detected from a subject to be controlled, thus obtaining a first result value. The second computation unit is configured to perform a second computation on the detected value, thus obtaining a second result value which is to be determined if equal to the first result value. The output control unit is configured to output the first result value in a case that the second computation unit determines that the first result value is equal to the second result value, and not to output the first result value in a case that the second computation unit determines that the first result value is not equal to the second result value.