A graph estimated to represent hepatic veins is extracted from image data. Shape models representing partial tree structures branched from a point of origin within a tree structure representing a common shape of hepatic veins are obtained. A cost function correlates the graph with the shape models. Whether parts extending from the peaks of correlated graph parts and not correlated with a shape model exist within the extracted graph is judged. Positional data of the peaks are obtained as data representing the position of the point of origin in the case that no such parts exist, or positional data of a node closest to an estimated position of the point of origin, specified by tracing along the nodes of such parts from the peaks to approach the estimated position, is obtained as data representing the position of the point of origin, in the case that such parts exist.
