PURPOSE: To eliminate the generation of reading error of the magnetic signal due to the overlap of recessed parts over the whole area of the magnetic signal even in the case where the stripe irregularity is formed on the back surface due to the vibration of a rotary grinding wheel and the looseness of a machine.

CONSTITUTION: In the case where a telephone card C is carried in the direction D by a rubber roller 1 and the printing on the back surface CB is ground for elimination by rotary grinding wheels 2A, 2B, 2C, axis of rotation of each rotary grinding wheel is set so that it does not cross the aligning direction of the magnetic signal D, which is recorded on the back surface CB, at a right angle, especially, in the case of the rotary grinding wheel 2A, axis of rotation of the rotary grinding wheel 2A is set in parallel with the aligning direction of the magnetic signal D.