PURPOSE: To attain ease of failure analysis, by displaying sequentially the transition of transmission coupling state to a specific position of a CRT screen to confirm visually and quickly the transmission coupling state when a transmission failure occurs.

CONSTITUTION: A central control section 4 controls a transmission control section 4 by the instruction of a program stored in a program storage circuit 6 and transmits/receives data according to a specific transmission means with an information processor 2 connected via a communication line 3 according to the specific transmitting procedure. A display device 1 has some transmitting coupling states in the process controlling the transmitting means in the process controlling the transmitting procedure and transits sequentially attended with the generation of cause to state transition such as transmission/receiving of data. The central control section 5 detects the transition of the transmission coupling state and displays it on a specific area 12 of the screen of a CRT10. Thus, the transition of the transmission coupling state of the display device 1 is confirmed visually.