PURPOSE: To secure the time for executing the evaluation by a host computer which is operating already by executing the evaluation of an evaluation program by executing the processing with respect to data sent to an evaluation system host computer through a communication system evaluating device.

CONSTITUTION: A work program 21 on an existing system host computer 20 connected to a circuit 44 through a circuit connecting part 22 from a transceiver 23, and a terminal 30 connected from a transceiver 31 execute a data communication work, while executing transmission and reception of data. Subsequently, an evaluation system host computer 40 connected to a circuit 44 through a circuit connecting part 42 from a transceiver 43 and loaded with an evaluation program 41, and a communication system evaluating device 10 connected to the circuit 44 through a circuit connecting part 15 from a transceiver 16 evaluate the evaluation program 41. In such a way, the evaluation can be executed without exerting the influence at all on its work in the course of operation of a regular work, and the evaluation can be executed by using its real data in the course of operation of an actual work, therefore, various evaluations can be executed enough.