A warning device according to the present invention includes an image acquisition unit configured to acquire an image group including a plurality of images taken in succession, a detection unit configured to perform detection of an object on each of images in the image group, a determination unit configured to compare detection results of the object in the image group in chronological order and determine a degree of movement of the object, and a warning unit configured to issue a warning in accordance with the determined degree of movement, wherein
the image acquisition unit acquires a plurality of image groups respectively based on a plurality of filter characteristics and the determination unit compares corresponding images between the plurality of image groups and thereby selects an image with high detection accuracy for each time period, compares the selected images in chronological order, and determines the degree of movement of the object.