PURPOSE: To simplify the processing and to eliminate malfunction, by receiving a signal decided optionally with an opposite station side, and processing automatically a data or folding the data.

CONSTITUTION: Whether or not a receiving data has a folding indication signal decided with the opposite station in advance is discriminated at a transmission control section 103, and when the indication signal is not included, the transmission control processing determined with the opposite station is performed. Further, a switching circuit is selected to the folding side. Then, the data transmitted from the opposite station are divided into those entering the control section 103 via a receiving section 5 and those folded at the switching circuit 104. When no folding signal exists in the receiving data, the control section waits for the processing so as to receive a new data and when the folding signal exists, the processing selecting the switching circuit 104 from the folding side to the normal data processing side is performed. Thus, the data processing and diagnosis are attained automatically.