1. A method for estimating three-dimensional (3D) positions of human joints using a sphere projecting technique, comprising the steps of: a) marker-free motion capturing a moving figure for obtaining a multiview 2D image of the moving figure, and extracting a 2D feature point corresponding to a bodily end-effecter from the obtained multiview 2D image of the moving figure; b) three-dimensionally matching the 2D feature point corresponding to the bodily end-effecter, and recovering the 3D coordinates of the bodily end-effecter; c) generating a 3D blob of the bodily end-effecter by using the recovered 3D coordinates, generating a virtual sphere with a radius that is a distance from a center of the 3D blob to a joint, and projecting the virtual sphere onto the obtained multiview 2D image of the moving figure; d) detecting a coinciding point of a surface of the projected virtual sphere and the multiview 2D image of the moving figure, and estimating a 3D position corresponding to the coinciding point as a 3D position of the joint; wherein the step a) comprises the steps of: a1) calculating mutual positions, dispositions, and focal distances of each of a plurality of cameras positioned at different angles, and calibrating the plurality of cameras according to measured criteria; a2) marker-free capturing the moving figure by using the plurality of cameras and obtaining a multiview 2D image of the moving figure, and extracting 2D feature points of the bodily end-effecter from the obtained multiview 2D image of the moving figure; and a3) calculating a joint distance from a center of the bodily end-effecter to the joint, obtained from the multiview 2D image of the moving figure; and wherein the step a3) comprises the step of measuring an arm reach length corresponding to a length between centers of either hand of outstretched arms of the moving figure from the obtained multiview 2D image of the moving figure when detecting the 3D position of an elbow joint from the human joints, and calculating a distance between a center of a hand and an elbow joint thereof using a ratio of the measured arm reach length to a length of the body.