1. An image processing apparatus, comprising: a display unit configured to display an image of a measurement object and an image of an object which corresponds to the measurement object and has a 3-dimensional (3D) shape which is calculated in advance; a designating unit configured to designate first points on the image of the measurement object and second points on the image of the object having the 3D shape based on an instruction input through an input device, the first points including three or more points, and the second points including three or more points; a calculating unit configured to perform a calculation of a rotational angle by which an angle of view of at least one of the image of the measurement object and the image of the object having the 3D shape is to be adjusted to thereby adjust a pose or a position of said at last one of the image of the measurement object and the image of the object having the 3D shape, wherein the calculating unit performs the calculation of the rotational angle by performing a geometric calculation to achieve a minimum difference between (i) a first vertex angle included in a first figure formed by the first points designated on the image of the measurement object and (ii) a second vertex angle included in a second figure formed by the second points designated on the image of the object having the 3D shape, wherein the second vertex angle in the second figure corresponds to the first vertex angle in the first figure; an adjusting unit configured to adjust the pose or the position of said at least one of the image of the measurement object and the image of the object having the 3D shape at least by adjusting the angle of view by the rotational angle that achieves the minimum difference between the first vertex angle and the second vertex angle, according to a result of the geometric calculation; and a measuring unit configured to calculate spatial coordinates on the object corresponding to a measurement position designated based on an instruction input through the input device after the pose or the position is adjusted, and to calculate a size of the measurement object based on the calculated spatial coordinates.