A magnetic tape library apparatus having mechanisms for reengaging a disengaged leader block of any magnetic tape cartridge entered. The apparatus comprises a cell unit having a plurality of cells each accommodating a magnetic tape cartridge, a magnetic tape drive unit for writing and reading data to and from the magnetic tape in the magnetic tape cartridge, and an accessor for automatically switching magnetic tape cartridges between the cell unit and the magnetic tape drive unit. The magnetic tape library apparatus further comprises a stacker for stacking a plurality of magnetic tape cartridges entered externally, and a delivery mechanism for selecting one of the magnetic tape cartridges stacked in the stacker for delivery to the accessor. Adjacent to the delivery mechanism is a hammer mechanism that hits the leader block for engagement with a latch.