PURPOSE: To set automatically a receiving Baud rate without recognizing a transmission Baud rate, by transmitting first a prescribed code signal from a transitter, discriminating the said code signal as the transmission Baud rate at the receiving side, and making the Baud rate of a signal processing circuit coincident with the code signal.

CONSTITUTION: The prescribed code signal is transmitted as the 1st transmission signal, and a receiver 2 reads the signal Vc with a clock CL1 in 9,600 Baud into a shift register 11. The signal Vc is inputted repetitively to a receiving terminal RxD of a USART13, and clocks CL1, CL2, CL3... are switched sequentially at each input of signals Vc', Vc".... When the transmission Baud rate is 4,800Baud, even if the signal Vc' is reproduced at the USART13 with the clock CL1 in 9,600 Baud, since the signal is different from the prescribed code signal, a CPU15 discriminates it as the dissidence in the Baud rate and switches the clock into the next clock CL2. Since the Baud rate is coincident in this case, the prescribed code signal is reproduced from a code reproducing section 12 so as to confirm the coincidence of the Baud rate. The succeeding signal processing is performed by fixing the clock of the USART13 to the CL2.