PURPOSE: To make it possible that even when a ring-shaped ferrite core has a large diamter and a small wall-thickness, its warp and deformation are hardly generated, and its product (sintered product) of a high roundness is obtained.

CONSTITUTION: A ring-shaped ferrite core compact 10 is burned, putting it on a base plate 12 which is made of the ferrite of the same material series as the compact 10 and is not sintered yet. The base plate 12 is put on a flat internal insulation 14. It is not necessarily required that the internal insulation is covered extensively with alumina grains or an alumina coating layer. When burning the ferrite core compacts 10 by piling up them to many stages, the core compacts of the same kind are prevented from seizing each other, by making the grains of alumina or zirconia, etc., lie between them. It is desirable that the molding density of the base plate 12 is made smaller than the ferrite core compact 10.