the magnetic field is changed from the rotating to the direct magnetic field hereinafter referred to as 34 change over temperature 34 as a parameter for the sake of comparison the change of the mr ratio of the mr element subject to the conventional discontinuous double step heat treatment i e the heat treatment in the direct magnetic field after the mr element is fabricated and the treatment in the rotating magnetic field after the in element is fabricated is shown by a broken curve a curve a shows the mr ratio for a change over temperature of 300 u00b0 c a curve b shows the mr ratio for a change over temperature of 250 u00b0 c a curve c shows the mr ratio for a change over temperature of 200 u00b0 c it will be noted that the mr ratio changes with the soaking temperature at which the mr element is heat treated in the rotating magnetic field and with the change over temperature according to the present invention the mr ratio is prevented from lowering more effectively than by the prior art the present invention suppresses lowering of the mr ratio by applying the direct magnetic field from during midway the cooling process a higher mr ratio is realized using a lower soaking temperature and a lower change over temperature referring now to fig3 the relationship is shown between the soaking temperature and the magnetic permeability with the change over temperature as a parameter a curve a shows the permeability of the in element for a change over temperature of 300 u00b0 c a curve b shows the permeability for.