In a storage system having a plurality of virtualization apparatuses that allocate a storage area which a storage device has, form a plurality of virtual volumes, and process input-output from a host processor to one of the virtual volumes, a request for completing all the input-output being processed that is received from the host processor and temporarily holding the input-output processing received subsequently is issued to the plurality of virtualization apparatuses, when a completion report of the processing of the input-output in response to the request is received from the plurality of virtualization apparatuses and the completion report is received from all the virtualization apparatuses to which the request was issued, an instruction of an allocation change of the storage area of the storage device is sent to all the virtualization apparatuses, and when the completion report of the allocation change is received from all the virtualization apparatuses, the storage area of the storage device is allocated to the virtual volume by sending an instruction for releasing a state of the input-output held temporarily to all the virtualization apparatuses.