To automatically set a communication condition when executing serial communication among a plurality of information equipment and to change the communication condition corresponding to the transaction of data even during the communication.

When setting the communication condition before starting the communication, a transmitting side communication device 10 sends out a communication condition change request signal. A receiving side communication device 20, receiving this communication condition change request signal, is turned into communication condition setting mode. Continuously, the transmitting side communication device 10 sends a data pattern for setting communication condition and the receiving side communication device 20 analyzes this data pattern. In the receiving side communication device 20, the communication condition is determined from this data pattern, and setting of the communication condition is automatically executed. Besides, by sending the communication condition change request signal during the communication, the communication condition can be changed during the communication as well.