A failure determination device allows a user to define conditions indicating a failure of the device. The failure determination device includes a difference detector configured to output a difference detection signal that indicates a difference between an input signal and an output signal output from a processor that performs a prescribed process on the input signal and a determination unit configured to output a determination signal indicating a determination result on presence or absence of a failure in the processor, based on the difference detection signal. A level detector outputs a level detection signal indicating whether a level of the input signal is within a prescribed range. The determination unit updates the determination signal when the level detection signal indicates that the level of the input signal is within the prescribed range, and stops updating the determination signal when the level detection signal indicates that the level of the input signal is not within the prescribed range.