A likelihood of a cross segment is calculated based on a cross segment characteristic condition defining a characteristic that a portion corresponding to an intermingled portion of a structure is present as a cross segment, and that at least two pairs of segments connectable in a straight line are present in the neighborhood of the cross segment. A likelihood of a straight line representing a probability that each segment is connected, in a straight line, to another segment is calculated based on a straight line connection condition defining a characteristic that each segment is connected, in a straight line, to another segment in the neighborhood thereof. A strength of connection between the segments is set based on the likelihood of a cross segment and the likelihood of a straight line, and plural network structures are constructed by connecting the segments based on the strength of connection.
