The magnetic tape library device is configured, including: a front magazine and a rear magazine for housing a large number of magnetic tape cartridges; a device main body in which the both magazines are housed in a parallel state in a freely slidable manner; an accessor mechanism for transporting the cartridge taken out from the magazines; and a magnetic tape drive which loads the magnetic tape cartridge and reads/writes data. A magazine pullout mechanism for pulling out the rear magazine and a magazine extracting mechanism for extracting the rear magazine from the device main body are provided.