1. An image processing apparatus comprising: a data obtaining unit configured to obtain volume data that contains a target image, the volume data including voxels; a depth-data obtaining unit configured to detect a surface of the target image based on the amount of reflection of light incident to the voxels included in the volume data and obtain depth data that indicates a depth to the surface of the target image from an image plane; an image processor configured to divide the volume data into a target volume and a non-target volume on the border of the surface of the target image obtained based on the depth-data, obtain processed volume data formed by removing the non-target volume from the volume data, and obtain a rendered image based on the processed volume data; and a display configured to display the rendered image, wherein the removing the non-target volume from the volume data includes altering each value of the plurality of voxels included in the non-target volume to a reference value.