By comparing an image of an observation object (S) acquired at each photographing timing with a previous image acquired at a photographing timing that immediately precedes the photographing timing at which each image has been acquired, the determination as to whether or not there is a change from the previous image in the observation object (S) is performed for each image acquired at each photographing timing. Then, by adding predetermined identification information to either an image for which a change from the previous image in the observation object (S) has been determined to be present, or an image for which a change from the previous image in the observation object (S) has been determined to be absent, it is possible to extract only an image in which a change from the previous image in the observation object (S) is present from among the saved images, on the basis on the identification information.