A calibration method for protrusion detection head units including steps of selecting as a master head unit a protrusion detection head unit of which lifting characteristic is known, further selecting one of any protrusion detection head units which is used for protrusion inspection as a measurement head unit, determining a disk having a predetermined surface roughness as a reference disk, successively loading the head of the master head unit and the head of the measurement head unit on the face of the reference disk which is rotated and causing to lift the same, measuring output voltages em of the master head unit and output voltages es of the measurement head unit while varying the circumferential speed of the track on the reference disk to determine as a function respective output voltage characteristics using the circumferential speed and linear-approximating the respective output voltage characteristics on a rectangular coordinate system of output voltage-circumferential speed to determine linear equations of em=-av+b and es=-a'v+b', wherein a and a' are inclinations and b and b' are intercepts on the output voltage axis of the linear equations.