Provided is a library device configured to comprise a magazine capable of containing a plurality of cartridges and a housing capable of containing the magazine in a two-tier manner, wherein: the housing is provided, in its inside, with an accessor comprising a picker unit for holding the cartridge and a base unit for moving the picker unit in the direction of insertion and drawing of the magazine and in the direction of tier arrangement; the magazine has a step member with a step formed therein; the housing has a locking member comprising a stopper part, which is disposed in a rotatable manner and positioned to abut the step, and a releasing part, and the housing has also a biasing member for biasing the stopper part in the direction of the step member; and, in accordance with movement of the picker and base units in the direction of drawing the magazine and movement of said picker unit in the direction of magazine-tier arrangement, unlocking of the upper-tier magazine, that of the lower-tier magazine or that of both of the magazines contained respectively in the upper and lower tiers is performed.