A failure determination device for an angle detector for a rotating machine, including: an angle detector, which includes n pole pairs (n>m), and outputs sine and cosine wave-signals in accordance with a rotational position of a rotating machine including a multi-phase winding and m pole pairs (m≥2); and a failure determiner for detecting failure of the detector based on signals from the detector. The detector outputs first and second sine-wave-signals having 180° phases different, and first and second cosine-wave-signals having 180° phases different. The failure determiner determines malfunction of the detector when the failure is determined with use of at least one of: failure determination using a sum of the first and second sine-wave-signals and a sum of the first and second cosine-wave-signals; or failure determination using a sum of squares or a square root of a sum of squares of the sine-wave-signal and the cosine-wave-signal.