1. An image processing apparatus comprising: an unit which acquires a tomographic image of an object obtained by an image sensing unit; an unit which acquires position and orientation information of the image sensing unit; an unit which acquires three-dimensional volume data of the object; an unit which acquires an image of a slice corresponding to the tomographic image from the three-dimensional volume data based on the position and orientation information; a generation unit which generates a composite image by composing the image of the slice with the tomographic image; and an output unit which outputs the composite image, wherein said generation unit obtains, for each pixel of the tomographic image, an image sensing clarity that takes a higher value corresponding to a lower pixel value, and composes the tomographic image with the image of the slice by composing, for each pixel, the tomographic image with the image of the slice in accordance with a ratio represented by a corresponding image sensing clarity.