PURPOSE: To eliminate the need for the manual setting of a parameter by sending a specific character from the sender side so as to inform a baud rate and a data format at the sender side to the receiver in advance and matching them from the sender side with those of the receiver automatically.

CONSTITUTION: A sender side sends it that a character sent next is a specific character for detecting a parameter with a break signal to a receiver side. The receiver side uses a timer of a CPU to count a pulse width of 'L' when a start bit 1 comes from the sender side. The receiver side decides the baud rate of the sender side depending on the numeral. In the case of detecting the data format, the data of the succeeding one bit is read by the CPU at the receiver side after a time x/2 from a point of time 3 when the level reaches 'L' again. Then the data format is decided from the read data. Thus, the communication parameter comprising the band rate and the data format is detected automatically. Thus, it is not required to set the communication parameter manually.