Faulty engagement due to low surface precision of the engaging surfaces of a center core of a disk cartridge and a drive spindle is resolved. The center core 10 is positioned on the drive spindle 20 in a state where the center core 10 is in point contact with the drive spindle 20 at a minimum of three spots upon chucking. A minimum of three protrusions 10d are provided on the bottom surface of the center core 10, and lower ends P of the protrusions 10d are in point contact with the upper surface 21a of the drive spindle 20.