To provide a load balance method in a virtual library system with which load balance is optimally set and backup of data is performed within a backup window time zone.

The load balance method for performing load balance in the virtual library system 10, comprises a first step for setting backup window information including time zones of backup for every combination of a host 1 and a virtual drive and backup capacity and calculating required average performance at the time of executing the backup for every combination of the host and the virtual drive based on the backup window information and the backup capacity, a second step for calculating whole activity ratio by every predetermined time unit based on a value obtained by adding the required average performance for every predetermined time unit for every combination of the host and the virtual drive and a third step for shifting a backup load allocated in a time zone with the maximum activity ratio to a time zone with the minimum activity ratio obtained in the second step for every time unit and performing load distribution so that the whole activity ratio to be calculated in the second step becomes equal to or less than the predetermined value when the activity ratio obtained in the second step exceeds a predetermined value.