1. A pose estimation method by using a plurality of pose candidates, comprising the steps of: a) formulating 3D model data indicating a three-dimensional shape of an object and reflectivity or color at every point of said object; b) creating, for each of said pose candidates, an image space representing brightness values of a plurality of two-dimensional images of said object which is placed in the same position and orientation as said each pose candidate, wherein said brightness values would be obtained if said object is illuminated under varying lighting conditions; c) detecting, for each of said pose candidates, an image candidate within said image space by using said 3D model data and determining a distance from the image candidate to an input image; and d) selecting one of the pose candidates which corresponds to the image candidate whose distance to an input image is smallest.