PURPOSE: To provide the adapter capable of recording or reproducing a different discoid recording medium in diameter with the disk device for such a medium without taking out the discoid recording medium from its disk cassette.

CONSTITUTION: A cassette housing part 31 for housing the disk cassette for 8cm CD is provided in a case main body 201. A shutter opening lever 32 is provided on the rear part side of the housing part 31. The size of a tabilizer (upper part side disk pulley) 212 is made to be corresponding to an opening part of the disk cassette for 8cm CD. Other constitution is made similarly to a caddy for 12cm CD. At the time of housing the disk cassette in the housing part 31 of the adapter 30, a shutter plate 203 of the disk cassette is pressed by the opening lever 32, and is brought into its completely opening position when the housing is completed. Under these circumstances, the disk cassette is loaded into the disk reproducing device for 12cm CD with its disk pulley diminished in diameter in corresponding to the opening part of the caddy for 8cm CD, and the medium is reproduced.