Based on the position and orientation information of an ultrasonic probe, an image generation unit acquires, from three-dimensional volume data, an image of a slice corresponding to a tomographic image of an object obtained by the ultrasonic probe. An image composition unit composes the image of the slice with the tomographic image to generate and output a composite image. In this composition, a region of the tomographic image is specified as an unclear image sensing region, and the image in the unclear image sensing region is replaced with the image in a region of the image of the slice corresponding to the unclear image sensing region, thereby composing the tomographic image with the image of the slice.
