PURPOSE: To easily execute data communication even if a clock frequency and a data length are not previously set between a transmission side and a reception side by inserting a preamble bit including clock components one bit before the start bit and the stop bit of a start-stop synchronizing signal.

CONSTITUTION: A preamble bit generation means 15 is provided in a start-stop synchronizing signal generation circuit so as to insert the preamble bit including the clock components one bit before the start bit and the stop bit. Since the reception side can easily decide the clock frequency and the data length by detecting the preamble bit, the data communication can be possible even if they are not previously set on the transmission side and the reception side. Thus the communication between equipments whose data lengths are different can be easily executed.