1. A shape-extraction method comprising: (1) a step of receiving image data of a projective image that has been acquired by radiation projection to an object; (2) a step of using a predetermined mesh structure to acquire a cross-sectional image of the object from the image data of the projective image by reconstruction using tomography; (3) a step of causing a lattice point that forms the mesh structure to move in conformity with the surface shape of the object based on the cross-sectional image that has been acquired by the reconstruction; and (4) a step of carrying out step (2) using a mesh that has had the position of the lattice point corrected using step (3), and then repeatedly executing step (3) and step (2) as many times as required.