An apparatus having a magnetic head assembly including a plurality of arms each arm having a magnetic head fixed to a nose portion thereof and a magnetic disk unit having a plurality of magnetic disks mounted on a rotating shaft. The spacing between adjacent magnetic disks being set at a value corresponding to the thicknesses of the magnetic heads fixed to two of the arms, plus the thickness of the two arms, plus a rotating gap of the magnetic heads, plus a space of 0.1 mm or less. Further included is a disk height measuring device and a head measuring device for measuring the heights of the disks and the magnetic heads. An expansion device expands the spacing between the adjacent arms. The spacing between the adjacent arms being expanded an appropriate amount to provide optimal operation of the magnetic disk apparatus.