PURPOSE: To position a real error generating track on a magnetic disk for which the number data of the error generating track can be provided by an error measuring instrument.

CONSTITUTION: At a main body 10 of this device, a case 2 of a magnetic disk 1 is fixed, and a window part 3 of the case 2 is exposed from an opening part 15. A scale 19 is fitted to a slider 17 and at the start, an exponent part 19a is positioned at the track of number '0'. A stepping motor 22 is controlled by a controller 23 to let a feeding screw 21 rotate, the slider 17 screwed to this screw is moved along a guide rod 16, and the exponent part 19a of the scale 19 is positioned at the error generating track.