PURPOSE: To improve test efficiency for the automatic test medium switching method in library device.

CONSTITUTION: At the time of testing a library device, in the order of a driving cell 2 from which a cartridge type medium (1) is unloaded, two sets of driving cell addresses and the address of a user cell 1 for which the cartridge type medium (1) is moved are stored in a change table 40 on a main storage device, a first and a second moved user cell addresses are switched on the change table 40, thereafter, the cartridge type medium (1) is moved from the user cell 1 of the address instructed by the switching table 40 to the drive cell 2 and the cartridge type medium (1) for a test, used by the drive cell 2 is changed. A read/write test by the drive cell 2 is executed random times, thereafter, by unloading the cartridge type medium (1), the order of the drive cell 2 to be unloaded is varied at random and the number of the drive cell 2 stored in the change table 40 is varied at random.