A surgery assistance apparatus includes an organ region extraction unit that extracts a tubular-organ region from a three-dimensional image obtained by imaging a tubular-organ and a blood vessel dominating the tubular-organ, a blood vessel region extraction unit that extracts a blood vessel region dominating the tubular-organ from the three-dimensional image, a branching structure extraction unit that extracts a branching structure of the blood vessel from the extracted blood vessel region, and a dominated region identification unit that identifies, with respect to an arbitrary partial blood vessel corresponding to an upper edge branching at least once to reach an edge including a terminal-end of the extracted branching structure, a dominated region in the tubular-organ region that is dominated by the arbitrary partial blood vessel by using positional relationships between a plurality of terminal end points present after the edge of the arbitrary partial blood vessel branches last and the tubular-organ region.