1. A slow change detection system comprising at least one central processing unit operating in conjunction with a memory to: acquire a photographed image including a plurality of consecutive images of a monitored object; acquire a reference image corresponding to the photographed image of the monitored object, the reference image having been photographed prior to acquiring the photographed image including the plurality of consecutive images; extract, from the reference image, a reference image area that is an area corresponding to a photographed area of the photographed image; identify a sudden change area in the acquired photographed image based on differences in the plurality of consecutive images of the photographed image; 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, the sudden change area; and superimpose and display information indicating the slow change area on the photographed image.