A computer system to prevent intervention and falsification by setting encrypted transfer between a host computer and a first storage device that provides a virtual volume and between the first storage device and second and third storage devices that provide a real volume corresponding to the virtual volume. A management computer specifies the second and third storage device that provide the real volume corresponding to the virtual volume by providing a volume corresponding to the virtual volume used by a host computer in which encrypted transfer becomes necessary, and setting the encrypted transfer to communication between the first storage device and the second and third storage devices, makes a reconnection thereof, and also sets the encrypted transfer to an I/O port used for the communication with the host computer in the first storage device.