A rib extraction unit extracts a rib from a three-dimensional image, a core line setting unit sets a core line of the rib, and a specific axis direction determination unit determines a specific axis direction in a cross section crossing the core line of the rib. An image generation unit moves a position of the core line in the specific axis direction according to an instruction to move the core line in the specific axis direction and generates a developed image of at least one rib based on a cross section along the core line at the moved position.
