An extraction unit extracts a target object from a three-dimensional image, and a feature point detection unit detects at least one feature point included in the three-dimensional image. A reference axis setting unit sets a reference axis in the three-dimensional image based on the feature amount, and a two-dimensional image generation unit generates a two-dimensional image by projecting a target object, which is included in the three-dimensional image, in a specific projection direction using the reference axis as a reference. A classification unit classifies each pixel of the target object into a plurality of classes based on the two-dimensional image.
