operations a plurality of times the characteristic whereby the shield magnetic domain is susceptible to changing can be made more apparent and the variation in the output voltage can be increased thereby making it possible to detect tested heads that are defective fig3 shows one example of how the output voltage of the tested heads 1 number of samples 2 vary when the intensity of the applied external magnetic field is changed from 0 gauss to 5000 gauss note that this data is for the case where the minute angle u03b8 described above is 7 6 u00b0 in fig3 the horizontal axis shows the intensity m in gauss of the applied external magnetic field and the vertical axis shows the variation the difference between the highest output voltage and the lowest output voltage in the output voltage expressed in u03bcv of the tested heads 1 when the magnetization operation and measurement of output voltage are repeated three times on the tested heads 1 and then the sensing current is reversed and the output voltage from the tested heads 1 is measured again as shown in fig3 when the intensity m of the magnetic field is in a range of 1500 gauss u2248 119 ka m to 5000 gauss u2248 398 ka m inclusive the variation in the output voltage of the tested heads 1 critically increases by using these characteristics even with tested heads 1 which have been determined to be non defective since no variation in output voltage was detected when testing using normal magnetization by setting the intensity m of the external magnetic field applied to the tested heads 1 at 5000 gauss for example measuring the output.