PURPOSE: To reduce a useless occupied time of a transmission line by sending a dummy data to a transmission line in detecting an error of an information block at a receiver side, detecting the collision between the dummy data and the data frame sent to the receiver side by the sender side, stopping the succeeding transmission, and retransmitting the information block not received normally.

CONSTITUTION: A data is sent to the receiver side via a transmission line in the unit of data frame at the sender side, and the data frame is received at the receiver side while the frame is divided into plural information blocks, the presence of an error is checked in each information block and in case of the presence of an error, the dummy data is sent to the sender side via the transmission line by the dummy data transmission means and the data is collided with the data frame sent from the sender side in the line and a collision detection means detects the collision at the sender side, then a normal information block sending means stops the transmission of the excess information block succeeding to the information block having an error, the information block not received normally is sent again and the reception end frame is transmitted from the receiver side to the sender side at the end of reception of a series or normal information blocks.