It is possible to consider an image as a composite wave that is the result of layering waves having differing periods and amplitudes. An image captured via a lens having a water droplet attached thereto has a higher occurrence of changes such as image blur than an image that does not have a water droplet attached thereto, which means that this type of change in an image results in changes in the composite wave thereof as well. Provided is a lens dirtiness detection device that suitably determines whether a lens is dirty without being affected by a background image by: focusing on the occurrence of large changes in the composite wave of an image that accompany an increase in the dirtiness of a camera lens; extracting each of the image frequency components that constitute the composite wave; and analyzing changes in the magnitude relation between the frequency components.
