PURPOSE: To test the whole input/output processor including the fault processing function of the interface part of the input/output device by conducting a test according to a test command when a test command receiving means receives the test command.

CONSTITUTION: A test start command is transmitted from a test command transmitter 12 to a communication control processor 30 to be tested through a test communication cable. The receiving device 33 of the communication control processor 30 which receives the test start command loads a test program to a processor 31 from a memory 32. The processor 31 sets a latch circuit in a connector 40 for adapter testing according to the test program and outputs test data to an external interface adapter 35. When the fault processing function of the external interface adapter 35 is tested, test data for generating a false fault is outputted. Therefore, the fault processing of the external interface adapters 35 and 36 can be tested.