The present invention provides a method for migrating from a source storage system to a target storage system. The method comprises the steps of: defining a volume defined on a drive to be migrated in the source storage system as an external volume of the target storage system; causing the host to access the volume on the drive to be migrated through an input/output port of the drive to be migrated as the external volume of the target storage system; blocking the other input/output port of the drive to be migrated while maintaining the access to the external volume of the target storage system; reconnecting the blocked input/output port with an interface in the target storage system; blocking the input/output port through which the external volume is being accessed, and connecting it withthe interface in the target storage system; and implementing the drive to be migrated in the target storage system. This method can be used to migrate data from the source storage system to the target storage system on a drive basis while receiving read/write requests for the data from the host computer and continuously using some of the source storage system.