The access from a host computer to a storage system is continued before and after data migration in the storage system. A first controller allocates a first logical storage area to a specific virtual storage area, stores first association information indicating the specific virtual storage area and the first logical storage area, and sends the first association information to the host computer. The first controller and the second controller perform data migration from the first logical storage area to a second logical storage area. The first controller releases the first logical storage area from the specific virtual storage area and reflects the release in the first association information. The second controller allocates the second logical storage area to the specific virtual storage area, stores second association information indicating the specific virtual storage area and the second logical storage area, and sends the second association information to the host computer.