[Aim] To construct a graph structure of bronchi in a three-dimensional medical image at higher accuracy [Solution Means] A continuous region extraction unit (32) extracts a continuous region having a voxel value corresponding to an air region in bronchi from a three-dimensional medical image. A tree structure construction unit (33) constructs a tree structure corresponding to the continuous region. Meanwhile, a linear structure extraction unit (34) extracts plural linear structures representing fragments of small bronchi by analyzing a local density structure in a neighborhood of each point in the three-dimensional medical image. A tree structure reconstruction unit (35) reconstructs a graph structure representing the whole bronchi by connecting a node constituting the graph structure of large bronchi and a node representing the linear structures of small bronchi. At this time, different cost functions are used for a segment connecting the node of the large bronchi and the node representing the linear structures of the small bronchi and a segment connecting the nodes representing the linear structures to each other, and the cost function in the former segment is defined in such a manner that the segment is more likely to be connected as a change in voxel values is smaller.
