Master node #0 and a plurality of slave nodes #1 to #4 are connected to one another via a plurality of optical couplers 20. Each of the nodes is controlled so that signal transmission between the nodes by a bus connection and a link connection is conducted via the optical couplers. In the link connection, the master node transmits bit signals corresponding to the number of the slave nodes. Each of the slave nodes transmits a bit signal with allocating the signal to a predetermined bit.