A library apparatus is capable of enhancing the dimensional accuracy of locker frame structures to allow an accessor to surely carry out the insertion/extraction of a cartridge into/from a storage unit or a deck. Each of the locker frame structures is made up of a top board, a base and sheet metal columns vertically standing between corner portions of the top board and the base. Each of the sheet metal columns is fixedly welded to the top board and the base from the inside of the locker. The library apparatus is applicable to a large-capacity external storage system storing a large number of cartridges such as magnetic tape cartridges and optical disk cartridges.