PURPOSE: To allow one set of communication controller to test an internal loop and an external loop by forming selectively the internal loop and the external loop in the communication controller, and replacing the own station address with an opposite station address.

CONSTITUTION: When an internal loop test instruction is issued from a processing program 11 of a CPU1A to an input/output instruction control block BK10 of the communication controller 2C, the BK10 indicates the internal loop control BK7 to be shifted to the internal loop. A table replacement BK9 replaces the received own station address with the opposite station address to an address section identification table BK8. When a test data is transmitted from an HDLC procedure control BK4C to a transmission control BK5C, the data returns to the BK4C via the BK7, arrow 7A and a reception control BK6C, and compared with a test data 12 at a CPU1A via the BK10, where the propriety is decided . In case of the external loop test, the BK7 is indicated to connect the address to a cable shwon in arrow 7B and the test is conducted while restoring the address received at the own station and the opposite station.