A disk apparatus compatible with a disk cartridge having a large head window even with a small external size.A disk apparatus includes a cartridge holder, a pair of guide walls, a disk motor, a head, a base, first and second slider guides, first and second open/close sliders respectively having first and second engageable portions. The slider guides are provided in the cartridge holder for supporting the disk cartridge, and the sliders are slidably engaged with the slider guides. The sliders slide to allow the engageable portions to be engaged with the disk cartridge such that a distance between the engageable portions is smaller than a distance between the guide walls while the disk cartridge is inserted, and such that the distance between the engageable portions is larger than the distance between the guide walls when the disk cartridge is mounted, and thus form a window in the disk cartridge so as to allow the head to access the disk and allow the disk to be mounted on the disk motor.