A storage device has a separate type magnetic head and a discrete type magnetic recording medium on which recording tracks and recording disabled areas are alternately formed. An accurate offset of the magnetic head with respect to the magnetic recording medium is measured even if the widths of the elements of the separate type head differ. While changing the write position and read position of a magnetic recording head, a write element writes measurement data to a magnetic recording medium, a read element reads the written measurement data, and the signal quality is measured and the write offset value and the read offset value are calculated from the inflexion points of a contour line on the two-dimensional map on the measured signal quality. Even if the widths of the write element and the read element differ depending on the individual magnetic recording head, an offset with respect to the separated recording tracks can be accurately measured.