PURPOSE: To reduce the overhead by establishing/release of association and to decrease the residence of a transmission message.

CONSTITUTION: Whether or not a transmission association cue of a same destination is already in existence is discriminated (step 200) upon the receipt of a transmission request, and whether or not the number of messages in transmission waiting reaches a maximum number is discriminated as to the transmission association of the same destination (step 201). When the transmission association of the same destination is not in existence or even when the transmission association of the same destination is in existence and the number of messages awaiting transmission in the transmission association cue does not reach its maximum number, an idle transmission association cue is utilized.