A physical storage area that is allocated to an unused area of a virtual volume is removed. A management unit sends a request to a server computer to make every piece of data stored in a first logical volume migrate to a second logical volume. The server reads all the data out of the first logical volume and writes the data in the second logical volume. A storage system that includes the first logical volume and the second logical volume allocates a physical storage area to an area of the second logical volume where the data is to be written, and writes the data in the allocated physical storage area. The storage system then deletes the first logical volume.