To prevent a command from being unsmoothly issued when a communications channel (LUN) from a conventional initiator to a target enters communications disabled state for any reason there is provided, a data processing method for use between two nodes which are connected to a serial bus and perform communications by a plurality of logical communications units, in which, in the plurality of logical communications units, a first logical communications unit is used in data communications from an initiator to a target, a second logical communications unit is used in data communications from a target to an initiator, and a control command issued from the initiator to the target can be issued to and received by both logical communications units.