Guide bars are positioned to define the center of a magnetic recording medium by contacting the inner circumference of a circular hole of the magnetic recording medium. Through holes are positioned in a medium grasping portion of a lower hub at equal distances from the center of the lower hub. The guide bars are inserted into the through holes of the lower hub and engaged with the inner circumference of the circular hole of the magnetic recording medium mounted on the lower hub. The end portions of the guide bars are then pushed by an upper hub to which a double-faced adhesive member is secured, thereby moving said guide bars away from the surface of the magnetic recording medium and securing the double-faced adhesive member to the magnetic recording medium and the lower hub.