Abstract: A rotary disk including a recording and reproducing sheet, and a hub formed of hard material secured to a central portion of the sheet. The hub comprises a center opening adapted to receive a main spindle of a turntable of disk driving means, two planar surfaces arranged in an intersecting relation to each other and formed on a projecting wall portion of the center opening, and a resilient member located in a position facing the two planar surfaces for pressing the spindle against the two planar surfaces. Even if the recording and reproducing sheet of the rotary disk is a thin magnetic sheet, there is no risk of damage of the sheet. When the rotary disk is set on the turntable, the center of rotation of the rotary disk and the center of rotation of the turntable can be automatically brought into alignment with each other.