PURPOSE: To prevent violation against rules by suitably preventing it that a data whose length is less than the specified length is sent to a LAN transmission line.

CONSTITUTION: A collision detection circuit 5 sets an output of a D-FF 6 to an H level when two outputs or more of carrier sense circuits 1-3 reach an H level. Then when all outputs of the carrier sense circuits 1-3 reach an L level, an OR gate 4 is used to reset an output of the D-FF 6 to an L level. A reset pulse generator 8 outputs a reset pulse of a first come priority circuit 10 synchronously with the falling of the output of the OR gate 4 to reset the first come priority circuit 10. Moreover, a collision data generated by a collision data generator 9 is sent to collision display lines C1-C3 while an output of the D-FF 6 is at an H level.