PURPOSE:To set automatically an optimum value of an RTS constant establishing program by using the said program at the system used for the operation. CONSTITUTION:A proper frame is transmitted from a system 20, the transmitted frame is compared with a frame received reflectedly from an MODEM 10 so as to confirm whether or not the information is transmitted normally. The optimum value of a time constant turning off a transmission request RTS signal is obtained by deciding the limit of normal frame transmission by means of a method such as the dichotomy. The optimum value decided by the RTS constant setting program is written in a file having a communication utility used for the actual system operation by means of the RTS setting program in this way. Thus, the optimum time turning off the RTS signal is set by using the value written in the file in the communication utility.