1. A method of detecting motion of a particular part from a sequence of successive input images, comprising the steps of: detecting motion vectors of a plurality of parts of the input images by tracking feature points in the successive input images; and detecting the motion of the particular part by processing the motion vectors, the motion detection step including producing a histogram of the motion vectors, processing the histogram such that a mountain-like distribution segment each having a peak at which a frequency distribution of motion vectors has a maximum in the histogram are classified, and a mountain-like distribution segment corresponding to the particular part is detected, defining class information for each feature point to indicate whether each feature point belongs or does not belong to the mountain-like distribution segment detected in the histogram processing step, and calculating motion of the particular part by analyzing the motion vectors belonging to the mountain-like distribution segment detected in the histogram processing step, the histogram processing step including, detecting the mountain-like distribution segment corresponding to the particular part on the basis of the class information defined for feature points.