A main chassis supports a tray that carries a disk thereon and moves between a disk-discharging position and a disk-loading position. A motor is mounted on the main chassis and has a rotating shaft. A worm is formed on the rotating shaft. A drive gear rotatably is mounted on the main chassis and has a pinion gear and a worm wheel in mesh with the worm. The pinion gear is in mesh with a rack formed in the tray. The worm preferably satisfies a dimensional condition .gamma.o>Tan.sup.-1(.mu./Cos .alpha.n) where .gamma.o is a lead, .mu. is friction coefficient, and .alpha.n is normal pressure angle. The worm has a lead angle .gamma.o greater than 15.degree.. The rotating shaft extends in a direction at an angle with a plane in which the drive gear rotates.