1. An image processing apparatus comprising: a storage unit configured to store a volume data set containing a tubular structure in a predetermined coordinate system; an extraction unit configured to extract a centerline of the tubular structure from the volume data set; an image generation unit configured to generate data for a first curved multi-planar reconstruction (CPR) digital image for a first curved plane along the centerline and a second CPR digital image for a second curved plane along the centerline based on the volume data set; an image specification unit configured to select one of the first CPR digital image and the second CPR digital image that has an image length that is longer than the other one; and a display unit configured to display the selected one of the first CPR digital image and the second CPR digital image, wherein the image generation unit generates the first curved plane and the second curved plane based upon a plurality of cutting lines passing through points along the centerline respectively at a first predetermined angle and a second predetermined angle, wherein the image specification unit determines a first image length based upon the image length of the first CPR digital image along an axis that is perpendicular to a direction of the cutting lines at the first predetermined angle and a second image length based upon the image length of the second CPR digital image along an axis that is perpendicular to a direction of the cutting lines at the second predetermined angle, the image specification unit selecting a longer one of the first image length and the second image length.