1. A medical image processing apparatus comprising: a core line identifying unit configured to identify a core line of a predetermined region of a tubular structure from medical three-dimensional image data; a center position identifying unit configured to identify a center position of the tubular structure from a plurality of points forming the core line identified by the core line identifying unit; an obtaining unit configured to obtain positions where straight lines intersecting at the center position and the tubular structure in the medical three-dimensional image data are in contact with each other; and a calculating unit configured to calculate a length of the tubular structure based on the positions obtained by the obtaining unit.