1. A slow change detection system comprising: an image acquisition unit adapted to acquire a photographed image including consecutive images of a monitored object; a reference image acquisition unit adapted to acquire a reference image corresponding to the photographed image of the monitored object; a reference image area extraction unit adapted to extract, from the reference image, a reference image area that is an area corresponding to a photographed area of the photographed image; a change detection unit adapted to acquire a change area that is an area of the photographed image which is different from the reference image area and acquire a slow change area by excluding, from the acquired change area, a sudden change area derived from a history of the change area; and a display control unit adapted to superimpose and display information indicating the slow change area on the photographed image.