PURPOSE: To transmit data by completing automatic poll control when transmission data is generated in a communication controller during execution of the automatic poll control and a signal returned from a terminal device is information data.

CONSTITUTION: A communication controller 20 is provided with a transmission control circuit 21-3 controlling the outgoing side of a subchannel of a duplicated line, a reception control circuit 21-2 controlling the incoming side of the said subchannel and a reception signal identification circuit 21-1 identifying that the incoming reception signal is an information signal. The automatic poll control is finished even when transmission data is generated during the execution of automatic poll control in the polling and a signal returned from terminal devices S1∼S3 is an information data without F. Thus, when transmission data is generated during the automatic poll control and the information data without F is received from the devices S1∼S3, it is identified to complete the automatic poll control and the transmission data is transmitted to the devices S1∼S3.