The spare disk unit existing in the port other than the parity group to which a failure disk unit in a disk array belongs is most preferentially selected as an alternative destination and data is reconstructed. When a system is made operative, the spare disk unit is allocated to a different port position every rank and the spare disk unit belonging to the rank of the failure disk unit is most preferentially selected as an alternative destination. Due to this, the selection of the alternative destination by the spare disk unit is optimized and the deterioration of performance in data reconstruction is prevented.