To provide a magnetic medium processor for preventing the skew of a magnetic medium without damaging a magnetic medium, and for stably performing the reading/writing of magnetic information.

This magnetic medium processor is provided with a width narrowing mechanism 6 for moving a magnetic medium to a medium width direction orthogonal to a conveyance direction, and for pressing it to a reference face, and configured to convey the magnetic medium while pressing the magnetic medium to the reference face by the width narrowing mechanism 6 for performing the reading/writing of the magnetic information by the magnetic head. The width narrowing mechanism 6 is provided with a spacing plate 22 which moves to the width direction by a stepping motor 27, width narrowing plates 23 and 24 which move according to the movement of the spacing plate 22 and width narrowing levers 25 and 26 which are pulled by the width narrowing sprints 51 and 52 according to the movement of the width narrowing plate to move and press the magnetic medium. Then, after the magnetic medium is pressed with the first pressing force to the reference face, the magnetic medium is conveyed in such a status that the magnetic medium is pressed with the second pressing force whose scale is different from the scale of the first pressing force, so that the reading/writing of magnetic information can be performed.