PURPOSE: To prevent the in-line shift of a gap which is made easily during the polishing of a multitrack head and to perform work into a shape which provides an excellent tape touch by providing a grindstone with a specific magnetism pattern, and controlling automatically the in-line performance of the R-polished gap of the head.

CONSTITUTION: The grinding wheel 1 has the magnetism pattern as shown in a figure, the magnetic head 2 to be polished which is swiveled reciprocally with a radius R is supported through a supporting arm 6 opposite to the wheel 1, and the supporting arm 6 is driven by a motor 5 to control the left-right inclination of the head 2 in the rotating direction of the wheel 1. This results from that the in-line performance to a tape is excellent when a coil for each channel becomes parallel to the rotating direction of the wheel 1. Namely, outputs of both end channels of the head 2 are amplified by amplifiers 3 and 3 and respective waveforms are compared by a phase comparator 4 to calculate the phase difference; when there is the phase difference, a difference signal is supplied to a motor controller 7 to drive the motor 5 forward or backward, obtaining the in-line performance of the gap.