1. A spatial motion calculation apparatus comprising: a first input unit configured to input image positions, spatial positions, and feature descriptors of a plurality of first feature points within a tracked area of an object of interest before the object moves; a second input unit configured to input image positions, spatial positions, and feature descriptors of a plurality of second feature points within an area of containing the object of interest after the object has moved; an image position relation calculation unit configured to determine each of first similarity functions based on the image position of each of the plurality of first feature points and the image position of each of the plurality of second feature points, each of the first similarity functions (1) containing the shift on the image and a parameter representing the depth direction on the image as unknowns and (2) having a larger value when the distance between the image position of each of the plurality of first feature points and the image position of each of the plurality of second feature points is closer; a spatial position relation calculation unit configured to determine each of second similarity functions based on the spatial position of each of the plurality of first feature points and the spatial position of each of the plurality of second feature points, each of the second similarity functions (1) containing the shift on the image and the parameter representing the depth direction on the image as unknowns and (2) having a larger value when the distance between the spatial position of each of the plurality of first feature points and the spatial position of each of the plurality of second feature points is closer; a feature descriptor relation calculation unit configured to determine each of third similarity functions based on the feature descriptor of each of the plurality of first feature points and the feature descriptor of each of the plurality of second feature points, each of the third similarity functions having a larger value when the feature descriptor of each of the plurality of first feature points and the feature descriptor of each of the plurality of second feature points are more similar; an integration unit configured to determine the shift on the image and the parameter representing the depth direction on the image that maximize an integrated similarity determined by the sum of the products of the first similarity functions, the second similarity functions, and the third similarity functions; and a spatial motion calculation unit configured to calculate the spatial motion of the object of interest between the positions of the object of interest before and after the movement based on the shift on the image and the parameter representing the depth direction on the image.