Abstract: A continuous-region-extraction unit 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 constructs a tree-structure corresponding to the continuous-region. A linear-structure-extraction unit 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 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.