A tape library control apparatus includes a storage for storing configuration information, a diagnostic evaluator, and a command controller. A tape library apparatus includes may include a plurality of tape drives and a plurality of magnetic tapes which includes specific magnetic tapes assigned for respective pieces of data specified by each data request issued by a host computer and a common magnetic tape provided for writing the respective pieces of the data. The diagnostic evaluator determines a use mode of the tape library apparatus on the basis of the configuration information. When the total number of the specific magnetic tapes is higher than the number of available tape drives, the use mode is determined and a command is issued to the tape library apparatus from the command controller to write data to the specific magnetic tape and to write to the common magnetic tape the respective pieces of the data.