PURPOSE: To start the transmission preparation of a next frame without waiting for the transmission completion of whole data in one frame.

CONSTITUTION: At the time of transmitting a transmission data block prepared for a main memory 3 to a transmission line 15 through a buffer memory 4, a data transmission circuit 5 and a transceiver 14, a central processing unit 2 stores the head character of transmission data in a head character storage circuit 9. A character comparison circuit 8 compares a character constituting the transmission data block with the head character. The central processing unit 2 prepares the transmission of the transmission data block which is to be transmitted next at the time of time-out when a collision detection circuit 6 does not detect collision before the head character is detected and a collision window monitor timer 7 is timed out.