Disclosed is a disk adapter for receiving a disk-type recording medium on an inner circumference side thereof, comprising: a main plate portion having a substantially arcuate shape of a central angle of 180° or more; and a supporting portion formed on a half circumference side of the inner circumference of the main plate portion for supporting the disk-type recording medium mounted on the disk adapter. Consequently, mounting and removing operations of a disk-type recording medium onto and from the disk adapter are very simple.