A foreign substance detection device for limiting deterioration in accuracy of foreign substance detection even if a foreign substance detection region changes. The device includes an imaging device configured to keep a region for detecting a foreign substance in an imaging range of the imaging device and a determination unit configured to determine a presence or absence of the foreign substance on the basis of a result of comparing a captured image captured by the imaging device with a reference image free of the foreign substance in the imaging range. The determination unit determines that the captured image is free of a foreign substance on the basis of the result of comparing and a prescribed first condition is satisfied, the unit executes an update process of replacing the reference image with the captured image for which the unit determines freedom of a foreign substance as an updated reference image.