Deck mechanism assembly comprises deck base, head guide assembly which is disposed on the deck base and which reads and writes data from/to a magnetic tape, cartridge reel motor which drives a reel on which the magnetic tape is wound, clutch which transmits a drive force of the cartridge reel motor to the reel enclosed in the cartridge, loader which conveys the loaded cartridge and mounts the cartridge in the state of the magnetic tape being able to be pulled out, loader motor which drives the loader, threader which pulls out a leader block placed at an edge portion of the magnetic tape and conveys the leader block, and head brush mechanism which functions corresponding to the leader block which is conveyed. Since the loader motor is disposed in the rear of the loader on the deck base, the width of apparatus can be decreased.