PURPOSE: To remove the relation of master and slave devices from two devices under the same conditions, by sending a synchronization confirmation pattern for a fixed period at the completion of acquisition of synchronism of a self- device and then transmitting information at the detection of the synchronization confirmation pattern from the other device.

CONSTITUTION: Interfaces 2a, 2b transmit/receive a signal 5 indicating the completion of the setting of information 3, 4, clocks 6 and transmission lines 7, 8 to and from synchronization controlling circuits 1a, 1b. When receiving the signal 5, a receiving synchronization controlling circuit 15 generates out-of-sinchronization pattern sending indication signal 103 and receives a synchronization pattern from the opposite device simultaneously, and after completion of acquisition of synchronism, generates a synchronization confirmation pattern sending indication signal 104. On the other hand, a transmitting synchronization controlling circuit 12 sends the out-of-synchronization and synchronization patterns in accordance with the indication signals 103, 104.