1. An object tracker comprising: a sampling unit which generates particles based on an estimated existence probability distribution of a target object in a first image frame, the particles being used for a particle filter in a parameter space where target candidates are defined, the first image frame being contained in a video stream together with a second image frame; a plurality of model tracking units which correspond respectively to multiple motion models and observe likelihoods of the target candidates with respect to the target object in the second image frame, the target candidates being defined by the particles transited according to each motion model; and a tracking integration unit which chooses one of the motion models based on a comparison of the likelihoods observed by the model tracking units, and estimates an existence probability distribution of the target object in the second image frame based on the likelihood of a target candidate which is defined according to the motion model chosen.