To stably record and reproduce information by making the thickness of an inner peripheral part thicker than that of an outer peripheral part in a disk shaped plastic magnetic recording medium for recording information, thereby reducing deformation amount at the time of clamping the medium.

The inner peripheral part 3a of a plastic magnetic disk 3 is a part to be clamped by a clamper in order to make a spindle moter rotate the disk 3. The outer peripheral part 3b of the disk 3 is a recording part for recording and reproducing information. Then, the inner peripheral part 3a is made to be a projected shape in the vertical direction of the disk and the thickness A2 of the inner peripheral part 3a is made thicker when it is compared with the thickness A1 of the outer peripheral part 3b. Moreover, since the contact area of the inner peripheral part 3a is wider than that of a disk drive clamper, when the disk is clamped by the clamper, the deformation amount to the outer peripheral part 3b is minimized.