PURPOSE: To effectively utilize a full duplex communication line rationally by using a computer (that of a personal computer scale is assumed) in common or as a communication means for each equipment so as to use the simplified communication means in terms of the hardware.

CONSTITUTION: Computers 110, 210 (assumed as microcomputers or those with personal computer scale) are used as two communication equipments of 1st and 2nd equipments 100, 200 connected electrically via signal transmission lines 30a, 30b forming the full duplex communication line. A program is set to either of the computers at least such that input processing is applied to a transmission signal received from an opposite computer through interruption processing and the response representing the inputted signal in case of the reception is returned to the opposite computer, and as for the sent signal, the program is set in advance so that the same signal is sent repetitively till the response is returned from the opposite computer.