In a disk cartridge, a round portion with a circular arc shape whose center substantially coincides with that of a disk is formed at the forefront edge of the disk cartridge in the direction of insertion into a disk drive unit, and the remaining portion is a rectangular shape. The round portion preferably is formed so as to be decreased gradually in thickness toward its leading end. On the other hand, part of the components of the disk drive unit is arranged in an open space at corner portions at the forefront of the disk cartridge. A space in the disk drive unit thus can be utilized efficiently, whereby a small, thin disk drive unit having excellent operability and a disk cartridge permitting such a disk drive unit can be obtained.