PURPOSE:To insert and clamp the hub of a disk between the disk and a spring under specific pressure and to secure transmission of its rotation, by providing a holder with the spring for pressing and holding a magnetic disk cassette, and providing the spring with a rotating body which presses the hub. CONSTITUTION:A disk retaining mechanism 4 consists of the leaf spring 69 and a pad arm 70. The boss 71a of the rotating body is fitted in the through hole 69a formed in the center of the leaf spring 69 from below and fitted rotatably. The arcuate part 71b of the rotating body 71 enters into the through hole 54a of the base plate of the holder to secure the smooth insertion of the cassette 5. This rotating body 71 contacts the hub of the floppy disk in the cassette 5 to support the rotation of the disk from above. Spring pieces 75 and 76 provided to the leaf spring 69 press the cassette 5 from above.