1. An object tracking apparatus comprising: a candidate contour determining unit configured to determine a candidate contour of an object to be tracked in a second image frame based on an estimated existence probability distribution of the object in a first image frame, the first image frame and the second image frame being in an image stream, the image stream constituting captured moving image data on the object to be tracked; an observing unit configured to observe a likelihood of the candidate contour by matching the candidate contour determined by the candidate contour determining unit against an edge image of the second image frame; and a result acquiring unit configured to estimate the existence probability distribution of the object in the second image frame, based on the likelihood observed by the observing unit, the observing unit including: a plurality of search units configured to perform, in parallel and for each knot, a processing to search for a nearest edge from each knot constituting a curve of the candidate contour; and a likelihood acquiring unit configured to acquire the likelihood for each candidate contour by integrating the results of search obtained by the search units.