An arm member is rotated when a disk is inserted into a disk apparatus from a disk insertion port while a circumferential edge of the disk abuts on a protrusion of the arm member for intercepting misinsertion of the disk. The arm member detects whether the disk inserted into the disk apparatus is present. When the disk is stored in the disk apparatus, the protrusion is arranged near the disk insertion port by the rotation of the arm member. If a disk other than the disk stored in the disk apparatus is to be erroneously inserted from the disk insertion port, the arm member that detects whether the disk is present is used as means for intercepting misinsertion of the other disk.