1. A position/orientation measurement apparatus comprising: a model holding means for holding a three-dimensional shape model representing a geometric feature of an object; an approximate value acquisition means for acquiring an approximate value indicating a position and an orientation of the object; an image acquisition means for acquiring a two-dimensional image of the object captured by an image capturing apparatus; a projection means for projecting the geometric feature of the three-dimensional shape model on the two-dimensional image based on the approximate value, and calculating the direction of the geometric feature of the three-dimensional shape model projected on the two-dimensional image; a detection means for detecting an image feature based on the two-dimensional image and calculating the direction of the image feature; an association means for associating the image feature and the geometric feature by comparing the direction of the image feature calculated based on the two-dimensional image and the direction of the geometric feature calculated based on the three-dimensional shape model; and a position/orientation calculation means for calculating the position and the orientation of the object by correcting the approximate value based on the distance between the geometric feature and the image feature associated therewith by the association means.