A method of manufacturing master disks for magnetic transferring, by which a metal disk of a prescribed thickness is formed by electroforming over an original disk on which a convexo-concave pattern matching transfer information is formed, a master substrate is fabricated from the metal disk peeled off the original disk, and a magnetic layer is formed over the convexo-concave pattern of the master substrate, the method comprising the steps of: monitoring either continuously or at prescribed intervals of time the content concentrations in an electroforming bath used for the electroforming, and carrying out bath adjustment to maintain the content concentrations in the electroforming bath within a prescribed range.