To start a bulk synchronization sequence again from any of terminals by eliminating the need for setting of a control side and a controlled side so as to avoid disabled operation due to a set error.

A data terminal 1 is connected to a terminal interface 21, line control sections 24-26 are connected to leased high speed digital lines 3-5, and when a bulk control section 22 conducts bulk communication, delay between the lines 3-5 is corrected and scrambled '1' or '0' is sent/received to the high speed digital lines 3-5 to which a scramble transmitter-receiver 23 is connected. Thus, two terminals in pairs that conduct bulk communication by using a plurality of the leased high speed digital lines 3-5 can be decided for a control side and a controlled side depending on their operations.