PURPOSE: To conduct the full duplex operating test for a data tranasmitter with a minimum device constitution by connecting a full duplex operation testing device to the transmission line and returning a data from a data transmitter being a test object at an irregular time interval.

CONSTITUTION: A test frame is sent to a transmission line 3 via a data transmitter 2 according to the test program of an external device 1. In receiving an information frame from the transmission line 3, the full duplex testing device 10 stores the information frame for an optional period and sends back the data to the transmitter 2 via the transmission line 3. The test program of the external device 1 compares the information frame received by the transmitter 2 with a predetermined expected value to conduct the operation test of the data transmitter. In such a case, the transmission interval time of the information frame sent back to the transmitter 2 from the testing device 10 is made at random to conduct the test of the full duplex operation of the data transmitter 2.