A 3D result image of an object is reconstructed from a set of X-ray two-dimensional projections of the object. Next, an M layer, including a metallic element M that is at least one element selected from the group consisting of Al, Ga, In, Sn, Pb, Bi, Zn and Ag, is deposited on the surface of the sintered magnet body and then an RH layer, including a heavy rare-earth element RH that is at least one element selected from the group consisting of Dy, Ho and Tb, is deposited on the M layer. Thus, a difference between two input voltages of a differential amplifier increases. Numerous other aspects are provided.