1. An image capturing apparatus, comprising: an image input unit that inputs a time-series of image frames; a designating unit that receives designation of an initial tracking-target region that is a first region in a first image frame of the input time-series of image frames; a feature value extracting unit that extracts a feature value from a target region in the first image frame, wherein the feature value is an entropy of a color histogram of the target region in the first image frame; a first search unit that searches for a second region obtained by changing a size of the first region and by determining whether a feature value extracted from the second region satisfies a predetermined condition for enabling successful tracking, and sets the second region as a new initial tracking-target region; and a second search unit that searches for a region similar to the new initial tracking-target region as a tracking-result region in a second image frame subsequent to the first image frame of the time-series of image frames.