1. An image processing apparatus comprising: a storage unit configured to store a volume data set containing a tubular structure; an extraction unit configured to extract a centerline of the tubular structure from the volume data set; a generation unit configured to generate a plurality of image data sets concerning a plurality of curved planes based on the volume data set, each of the curved planes containing the centerline, and the plurality of image data sets having a plurality of image lengths respectively and containing a plurality of centerlines respectively; a first specification unit configured to specify a first image data set or a second image data set from the plurality of image data sets, the first image data set being specified based on the plurality of image lengths of the plurality of image data sets, and the second image data set being specified based on gradients of the plurality of centerlines of the plurality of image data sets; and a display unit configured to display the first image or the second image.