An image-capturing system diagnostic device includes: an image acquisition unit that obtains an image; and a monitoring unit that monitors a quantity of foreign matter present in an optical path by generating defect information indicating a defect at pixels caused by the foreign matter in the optical path based upon the image obtained by the image acquisition unit and calculating an areal ratio of defective pixels in the image based upon the defect information having been generated and issues a warning for a photographer if the areal ratio of the defective pixels exceeds a predetermined value.