A first control unit of a first control device executes: a synchronous copy process for writing data from a host device into a first storage medium, transmitting the data to a second control device, and replying to the host device; a storage process for storing the data written in the first storage medium into a first region, instructing the second control device on a buffer switch when a predetermined condition is satisfied, and switching a storage destination to a second region; and an asynchronous copy process for transmitting the data in the first region and the second region to a third control device at a time, respectively. A second control unit stores the data transmitted from the first control unit in a second storage medium and a third region, and when the buffer switch is instructed, stores the data in the second storage medium and a fourth region.