An image processing apparatus, comprising: a display unit (22, 25) 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 (R1, R2, R3) on the image of the measurement object and second points (R1', R2', R3') as 3D-points corresponding to the first points (R1, R2, R3) on the image of the object based on an instruction input through an input device, each of the first points (R1, R2, R3) and the second points (R1', R2', R3') including three or more points; a calculating unit (34c, 35c) configured to perform a pan/tilt direction-matching process; an adjusting unit (34c, 35c) configured to adjust a pose of the image of the object based on a result of the pan/tilt direction-matching process; and a measuring unit (34c, 35c) configured to calculate spatial coordinates on the object corresponding to each of at least two measurement positions designated based on an instruction input through the input device after the pose is adjusted, and calculate a size of the object based on the calculated spatial coordinates, characterized in that the calculating unit (34c, 35c) is configured to perform a calculation of a rotational angle in each of pan and tilt directions by: for a range of rotational angles in each of pan and tilt directions of the viewpoint with respect to the 3D shape, projecting the second points (R1', R2', R3') from the 3D shape onto the screen plane and calculating the vertex angles (A1, A2, A3) of a triangle configured with the projected second points (Rp1', Rp2', Rp3'), and then searching the pan and tilt angles for which the average difference of corresponding vertex angles between a triangle configured with the first points and a triangle configured with the projected second points is minimum. a display unit (22, 25) 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 (R1, R2, R3) on the image of the measurement object and second points (R1', R2', R3') as 3D-points corresponding to the first points (R1, R2, R3) on the image of the object based on an instruction input through an input device, each of the first points (R1, R2, R3) and the second points (R1', R2', R3') including three or more points; a calculating unit (34c, 35c) configured to perform a pan/tilt direction-matching process; an adjusting unit (34c, 35c) configured to adjust a pose of the image of the object based on a result of the pan/tilt direction-matching process; and a measuring unit (34c, 35c) configured to calculate spatial coordinates on the object corresponding to each of at least two measurement positions designated based on an instruction input through the input device after the pose is adjusted, and calculate a size of the object based on the calculated spatial coordinates, characterized in that the calculating unit (34c, 35c) is configured to perform a calculation of a rotational angle in each of pan and tilt directions by: for a range of rotational angles in each of pan and tilt directions of the viewpoint with respect to the 3D shape, projecting the second points (R1', R2', R3') from the 3D shape onto the screen plane and calculating the vertex angles (A1, A2, A3) of a triangle configured with the projected second points (Rp1', Rp2', Rp3'), and then searching the pan and tilt angles for which the average difference of corresponding vertex angles between a triangle configured with the first points and a triangle configured with the projected second points is minimum. for a range of rotational angles in each of pan and tilt directions of the viewpoint with respect to the 3D shape, projecting the second points (R1', R2', R3') from the 3D shape onto the screen plane and calculating the vertex angles (A1, A2, A3) of a triangle configured with the projected second points (Rp1', Rp2', Rp3'), and then searching the pan and tilt angles for which the average difference of corresponding vertex angles between a triangle configured with the first points and a triangle configured with the projected second points is minimum.