1. An image processing apparatus, comprising: a memory configured to store a volume image of a three-dimensional region of a subject; and processing circuitry configured to generate a plurality of slice images corresponding to a plurality of slices each substantially perpendicular to a predetermined reference axis from the volume image, extract a target region from the plurality of slice images, detect an end point from the extracted region, and extract a tubular structure based on the end point, wherein the processing circuitry is further configured to extract the tubular structure through obtaining a first fused part of a tree-shaped tubular structure by fusing a first tree-shaped tubular structure together with a first part of candidate data based on spatial continuity of the first tree-shaped tubular structure, the first tree-shaped tubular structure being obtained by using first region growing, and the candidate data being obtained by using grayscale morphological reconstruction, after obtaining the first fused part of the tree-shaped tubular structure, recognizing a twig of the obtained first fused part of tree-shaped tubular structure, performing second region growing by using the twig to obtain an extended branch, and obtaining a second fused part of the tree-shaped tubular structure by fusing the extended branch together with a second part of the candidate data based on spatial continuity of the extended branch.