In order to rotate a magnetic disk (3) supported on a discoid center core (31) in a cassette shell (2), the core (31) is attracted to the magnet (14) for connection of a turntable (12). A magnetic head inserting hole (6) is provided in a side face (23) of the shell (2) and a center hole (31a) into which a centering pin (13) of the turntable (12) is inserted is provided at the center of the core (31). An annular attracted section (31b) the height of which is different in steps from the height of the hole (31a) is provided on the periphery of the core (31). The attracted section (31b) is attracted in a face-contacting state to the magnet (14) annularly provided on the peripheral side of the turntable (12). When the magnetic disk rotates at a high speed in the cassette shell (2), therefore, the attraction of the center core (31) by the magnet (14) is strengthened, the magnetic disk does not slip nor vibrate in case of an unbalanced load.