To develop a re-transmission mechanism for transmitting an erroneous message generated in a handshake or start protocol.

At the time of receiving an erroneous message in the xDSL negotiation procedure of a communication session, a signal and a message are minimized. A receiving part (52) monitors reception data related with the frame check sequence of an xDSL negotiation procedure. When it is decided that the erroneous message is received by the receiving part (52), a re- transmission requesting device (54) transmits a re-transmission request message. The re-transmission request message indicates whether or not an accurate message is received at last. When the preliminarily decided number of erroneous messages are generated, a communication section is stopped.