To enhance capacity expansion property of a storage system for providing thin provisioning logical volumes, a computer system includes a first storage system and a second storage system for providing a second volume to the first storage system. The first storage system is configured to define at least one first volume, define a first storage pool including the defined first volume and the provided second volume, and provide to the host computer a first thin provisioning volume that is recognized as a volume having a capacity equal to or larger than that of storage areas assigned from the first storage pool. The first thin provisioning volume is thus assigned with both storage areas of the first volume and storage areas of the second volume, which are included in the first storage pool.