Abstract: A method of testing bit errors in an ISDN circuit by employing a tester having the function of a terminal and by using the information channels of the frame of an ISDN. The tester sends out a known data pattern generated by a pattern generator to the ISDN by a sending circuit, receives the data pattern transmitted through the ISDN by a receiving circuit, and compares the known data pattern generated by the pattern generator and the received pattern data to detect errors in the circuit of ISDN. The method is capable of detecting errors in the circuit and confirming the quality of the circuit and the operating condition of the ISDN without requiring any additional special function of the circuit and without disturbing the communication of terminals connected to the ISDN.