A host I/F unit has a management table for managing an MPPK which is in-charge of the control of input/output processing for a storage area of an LDEV, and if a host computer transmits an input/output request for the LDEV, the host I/F unit transfers the input/output request to the MPPK which is in-charge of the input/output processing for the LDEV based on the management table, an MP of the MPPK performs the input/output processing based on the input/output request, and the MP of the MPPK also judges whether the MPPK that is in-charge of the input/output processing for the LDEV is to be changed, and sets the management table so that an MPPK which is different from the MPPK that is in-charge is to be in-charge of the input/output processing for the LDEV.