A disk unit comprising a limit switch, the limit switch being adapted to operate when a tip of a guide arm pivots to excess in a centripetal direction relative to a predetermined position at which it should stop while supporting a side portion of a small-diameter disk, the guide arm functioning to conduct a large- or small-diameter disk being conveyed to a predetermined position while supporting a side portion of the disk, a disk unloading operation being started upon operation of the limit switch to permit discharge of a disk of a deformed shape. The disk unit is a slot-in type disk unit permitting automatic loading and drive of two types of disks different in diameter, wherein a disk of a deformed shape inserted by mistake is discharged automatically to prevent damage of a recording surface of the disk and a mechanical damage of the disk unit.