In the polishing work for forming a crown shape on a ceramic bar bearing a plurality of conversion units for the magnetic heads, the present invention is to provide a well-controlled satisfactory shape by pressing the ceramic bar, provided with plural grooves perpendicular to the longitudinal direction and on a surface opposed to the polished plane, to a substantially concave polishing plane through an elastic member principally composed of rubber, then measuring the magneto resistivity of an element provided in the conversion unit of the magnetic head, and adjusting the pressure at the crown forming operation by a closed loop control based on the result of such measurement.