PURPOSE: To achieve monitoring without dropout of data such as a selection signal SS, by constituting that SSs and the like before the line probability running on a line can be monitored.

CONSTITUTION: A control section 310 can cope with various signal system with a stored program. The stored program stores all the signal systems of about 10 types and recognizes each data format and data pattern in advance. If the operator inputs what is one signal system now in use from the control section 310, from the console, in what timing and as to what data should run, the data can be grasped by the stored program completely. Further, whether the logic of data should be of A or Z polarity, can be grasped each of reception and transmission side. Thus, the drawing of data can be made independently and individually from the reception and transmission side.