PURPOSE: To provide an ISDN line monitor device which can automatically translate communication data on an ISDN (integrated service digital network) line into the screen of a display and can display it corresponding to a protocol actually used on the line.

CONSTITUTION: A protocol identification part 7 detects the frame type of a D channel link access procedure frame and the value of a service access point identifier. Based on the detected result, the kind of the protocol used on ISDN lines 10 and 11 is identified and corresponding to the identified protocol, the communication data on the ISDN lines 10 and 11 are automatically translated and displayed by a display part 8.