A magnetic disk cartridge which is used with a high density recording magnetic disk and which provides good recording and reproducing properties. The magnetic disk cartridge has a flexible magnetic disk having a disk-like center core. The upper surface of the disk-like center core is centrally affixed to the magnetic disk. The magnetic disk is rotatably retained in a flat housing. The magnetic disk has a surface recording density of at least about 158.7 Mbit/cm2. The disk wobbling value of the magnetic disk measured at a radial position located 5 mm outward from the outer diameter of the center core of the magnetic disk while rotating the magnetic disk by means of a drive device via the center core is not more than 20 nullm.