PURPOSE: To attain immediate emergency communication without reception of a token frame by sending a frame forcibly when the frequency to be sent exists on a transmission line so as to cause collision.

CONSTITUTION: When a data transmitter 2-1 desired emergency communication and a reception control section i-1 does not receive a token frame, while a frame is sent by other data transmitter 2-j on a transmission line under the control of a control section i-2, a new frame is sent forcibly from a transmission control section i-3. Then collision with a frame having been already sent is caused. Moreover, a collision detection section j-4 of other data transmitter 2-j detects the collision to interrupt the transmission, a transmission control section i-3 makes own emergency communication and a token frame is sent to abort the token after the end of emergency communication. Thus, even when no token frame is received, emergency communication is implemented immediately.