Recording medium changer and control method

A conveying block 20 for conveying recording mediums between slots 15 and a drive block 16 is provided with a recording medium delivery block 50 for delivering the recording mediums to and from the slots 15 or the drive block 16, an extendable and retractable rail portion for moving the recording medium delivery block to a desired slot, and a control block 17 for controlling extension and retraction operation of the rail portion 31. The control block 17 determines whether the slots 15 are provided in an expansion area provided in an extending direction of the rail portion 31, and when the slots 15 are provided in the expansion area, the rail portion 31 is extended so that the recording medium delivery block 50 is moved to the desired slot in the expansion area. The recording mediums are efficiently stored for changer operation.