a connecting part 31 c the two side plates 31 a and 31 b have a fan shape capable of covering half or more of the surface areas of both surfaces of the disk 1 and are spaced apart from each other while being positioned in parallel to each other the connecting part 31 c connects the two side plates 31 a and 31 b to each other while extending along the outer circumferences of the side plates 31 a and 31 b in such a way as to dose a gap between the two side plates 31 a and 31 b in this case the two side plates 31 a and 31 b cover the surfaces of the first and last disks 1 of several disks 1 installed on the spindle 11 respectively an opening is formed by the gap of the protective cover 31 in its reciprocating direction such that the cover 31 receives the disks 1 in the opening a partition plate 31 d is interposed in parallel with and between the two side plates 31 a and 31 b of the protective cover 31 to separate two or more disks 1 installed on the spindle 11 from each other when the cover 31 covers the disks 1 in the opening the partition plate 31 d also has a fan shape and is connected to the connecting part 31 c at its outer circumference the protective cover 31 protects the disk 1 rotating at a high speed from the flow of air surrounding the disk 1 that.