Extraneous-matter detecting apparatus and extraneous-matter detecting method

An extraneous-matter detecting apparatus includes a region setting unit, a computing unit, a change-value setting unit, an updating unit, and a detection unit. The region setting unit sets divided regions for a captured image captured by an image capturing device. The computing unit computes, for a corresponding divided region, a first difference between a past representative value of luminance and a present representative value of luminance and a second difference between a past dispersion amount of luminance and a present dispersion amount of luminance. The change-value setting unit sets, based on the first and the second differences, a change value of a counter value that indicates an adhesion state of an extraneous matter in the divided region. The updating unit updates the counter value based on the change value. The detection unit detects, based on the counter value, an adhesion state of an extraneous matter in the divided region.