Provided is a library device with which an accessor can move even when a magnetic tape cartridge is ejected out from a magazine onto a transporting path on which the accessor moves. The library device includes: a magazine inside which slots are provided; a device main body which houses the magazine; an accessor provided on one side of the device main body; and a magnetic tape drive which transports cartridges. A cartridge ejection preventing mechanism for preventing ejection of the entire part of the cartridge from the slot is provided in an opening-part front face of the slot, and a cartridge stopper member of the ejection preventing mechanism is set to be reciprocally rotatable along the moving direction of the accessor.