An overall image obtained by successively imaging different imaging regions a plurality of times during an image period using an imaging device is separated into a plurality of partial images respectively corresponding to each of the plurality of imaging operations. In cases where the object of imaging consists only of a static object, all of the partial images show more or less the same data. However, in cases where the object of imaging includes a moving object in addition to a static object, differences are generated in the partial images, which have different imaging timing. Accordingly, a judgement as to whether or not a moving object is contained in the overall image produced by synthesizing the plurality of partial images can be made by comparing the partial images.