. A retaining structure formed on a base of a disk holder, comprising:
a retainer including:
a plurality of elastic pieces equally spaced on the holding portion, the elastic pieces each having a peripheral end being a free end;
a pressing face positioned on a central portion thereof, and the pressing face connected to each of the elastic pieces through a connection part;
a trench formed along an peripheral side of the pressing face;
a plurality of wedges each formed on an external surface of each of the elastic pieces; and
a plurality of arcuate walls each formed between two adjacent elastic pieces and separated from each of the elastic pieces by a slit communicated with the trench; and
a recess formed on the base of the disk holder and opposite to the retainer.