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, characterized in that the change detection unit is adapted to divide the photographed image and the reference image area into subareas, determine whether or not the photographed image is different from the reference image area on a subarea basis and acquire, as the change area, an area of the photographed image including a difference subarea that is a subarea in which a degree of difference is large, define a difference subarea group surrounded by subareas that are not the difference subareas, as one object, and calculate a change amount of a center of gravity and an area in the object and, when the change amount is greater than or equal to a predetermined reference value, acquire an area including difference subareas forming the object as the sudden change area. 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, characterized in that the change detection unit is adapted to divide the photographed image and the reference image area into subareas, determine whether or not the photographed image is different from the reference image area on a subarea basis and acquire, as the change area, an area of the photographed image including a difference subarea that is a subarea in which a degree of difference is large, define a difference subarea group surrounded by subareas that are not the difference subareas, as one object, and calculate a change amount of a center of gravity and an area in the object and, when the change amount is greater than or equal to a predetermined reference value, acquire an area including difference subareas forming the object as the sudden change area.