An extraneous-matter detecting apparatus including a dividing unit that divides a captured image into a plurality of regions. The captured image is output from an image capturing device; a computing unit detecting luminance values of pixels included in each of the regions divided by the dividing unit, and computes, for each of target regions, an average value or a standard deviation of luminance values of the detected luminance values. The corresponding target region corresponds to the one or more regions; and a detection unit that extracts the target regions having the average values or the standard deviations of the luminance values computed by the computing unit, which are within a predetermined range, and detects, when the extracted target regions are adjacent to each other, the adjacent target regions as an extraneous-matter region. The extraneous-matter region is a region of the image capturing device to which an extraneous matter is adhered.