A first controller defines an existing pair that represents a copy from a first existing area that is a storage area of a first logical volume before size expansion to a second existing area that is a storage area of a second logical volume before size expansion, maintains the existing pair in a synchronous state, which is a state where the second existing area is in synchronization with the first existing area, and expands the size of the first logical volume by adding a first expansion area using a first storage device to the first logical volume during the synchronous state, and the second controller adds a second expansion area using a second storage device to the second logical volume after the size of the first logical volume is expanded, and writes data written to the first expansion area to the second expansion area.