Disclosed is a fault diagnosis device for diagnosing a fault in a bay Intelligent Electronic Device (IED) for a substation automation system, which includes an online IED diagnosis device and a backup IED. The online IED diagnosis device includes an IED selection unit, an IED switch unit, a fault diagnosis, and a result display unit. With reference to a Substation Configuration Description (SCD) file, the online IED diagnosis device designates an IED to be tested, and extracts backup information for the IED to be tested. A backup IED replaces the operation of the IED to be tested based on the extracted backup information. Further, the online IED diagnosis device determines the occurrence or non-occurrence of a fault in the IED to be tested by analyzing response information received in response to transmission of a test pattern to the IED to be tested.