1. An image processing device comprising: a controller configured to function as: a feature point acquiring unit configured to acquire a plurality of feature points of objects of each frame image, from frame images sequentially input from an imaging device; a grouping unit configured to perform a grouping process of dividing the plurality of input feature points into groups corresponding to individual objects, and then output the feature points; a masking unit configured to perform a masking process of excluding feature points included in some areas of the frame image, from the plurality of input feature points, and output feature point groups; a highlighting-image generating unit configured to generate highlighting images for highlighting objects of the frame image corresponding to the feature point groups if the feature point acquiring unit inputs the feature points to the grouping unit, and the grouping unit outputs the feature points to the masking unit, and the masking unit outputs the feature point groups to the highlighting-image generating unit; and a superimposed-image generating unit configured to superimpose the highlighting images generated by the highlighting-image generating unit, on the frame image, thereby generating a superimposed image, and output the superimposed image.