PURPOSE: To quickly restore system down abnormality due to the abnormality of a transmission line by detecting an abnormal position almost at real time when the abnormality is caused in the transmission line.

CONSTITUTION: The levels of signals outputted from RS485 type transmission lines TRX (+), TRX (-) connected to the output side of a personal computer 6 are discriminated by a comparator part 3 and whether the voltage levels of signals outputted from the lines TRX (+), TRX (-) are complementary or not is checked by a judging part 4 based upon a level discrimination result, and when both the voltage levels are complementary, the checked result is detected and the abnormality of the transmission lines TRX (+), TRX (-) is displayed by a display part 5.