PURPOSE: To measure a group delay without using a line monitor by obtaining the group delay from signals Q, I for eye pattern display and displaying the result.

CONSTITUTION: A MODEM 21 applying PSK modulation to a reception signal and applying FSK modulation to a transmission signal outputs two kinds of signals Q, I for eye pattern display. Various controls for transmission/reception and the operation processing to reproduce a picture according to the reception signal are executed by a CPU 22. Data are transmitted or received between the CPU 22 and the MODEM 21 by a buffer 23. The display control to display the group delay of the reception signal is executed by a display control circuit 24. The signals Q, I outputted from the MODEM 21 are not converted into an analog signal but fetched in the CPU 22 while being kept as a digital signal, the group delay is calculated and the display control circuit 24 drives a display circuit 25 according to the result of calculation to display the group delay.