1. A position and orientation measurement apparatus for measuring a three-dimensional position and orientation of a target object, a shape model of which indicates a three-dimensional shape of the target object and is given, said apparatus comprising: an obtaining unit adapted to obtain a range image to the target object, which is captured by an image capturing unit; a coarse position/orientation determination unit adapted to determine a coarse position and orientation of the target object based on the range image; a calculation unit adapted to calculate a variation degree of distance information on a region of the range image, which region corresponds to a predetermined region on the shape model; a sampling unit adapted to sample, for each predetermined region on the shape model, sample points from the predetermined region so as to reduce the number of sample points as the variation degree is smaller; an association unit adapted to associate the sample points and three-dimensional measurement points obtained by converting two-dimensional measurement points on the range image into three-dimensional coordinates based on the coarse position and orientation; and a position/orientation determination unit adapted to determine the position and orientation of the target object based on a sum total of distances between the sample points and the three-dimensional measurement points.