A slot-in disk device that is specific for applying in a 12 cm-disk. The disk device is capable of stopping an 8 cm-disk from entering the interior of the disk device; thereby the disk device can be free from failure. A disk supporting arm 17 is so arranged that, in a state when the disk supporting arm 17 is standing by for carrying a 12 cm-disk D into the disk device, a disk engaging section 33 located in a region where a first region, through which a 8 cm-disk Ds will pass when the 8 cm-disk Ds is inserted inwards along a first guiding section 27 from an intake 3a, overlaps a second region, through which a 8 cm-disk Ds will pass when the 8 cm-disk Ds is inserted inwards along a second guiding section 29 from the intake 3a, in order to preclude the 8 cm-disk Ds from being taken inside by mistake. In a state when the disk supporting arm 17 is standing by for carrying a 12 cm-disk D into the disk device, if an 8 cm-disk is inserted into the disk device by mistake, and the 8 cm-disk cannot be stopped from entering the interior of the disk device by the holder 18 of the disk supporting arm 17, then an engaging section 19a of a stopper 19 mounted at the front end of the disk supporting arm 17 will engage the front periphery margin of the 8 cm-disk Ds to hinder the 8 cm-disk from entering the interior of the disk device.