This storage system supplies, to a plurality of computers, a plurality of duplicate volumes (CVOLs) (corresponding to duplicates of a master volume (MVOL) upon which is stored an electronic object (EO) that is common to the plurality of computers). Both the MVOL and the CVOLS are virtual logical volumes that follow sync provisioning. In the plurality of CVOLs, a plurality of physical regions that are allocated to the MVOL (i.e. regions in which the electronic object is stored) (PAs) are allocated. A storage, when writing an electronic module (EM) to which the EO is applied to the first CVOL, copies data within a first PA that is allocated to the virtual region (VA) that is the write destination to a second PA, writes the EM to the second PA, and moreover allocates the second PA to a VA of the write destination, instead of the first PA. And the storage allocates the second PA to a VA within the second CVOL corresponding to the VA of the write destination, instead of the PA that is allocated to that VA.