PURPOSE: To reduce the circuit scale and the power consumption by using one transmission line quality discrimination series in time division corresponding to each input signal series.

CONSTITUTION: A signal selection circuit (a) selects an input interface signal 2 among inputted interface signals 11...1n with a series selection signal 8 generated from a control signal generating circuit (g). From the selected input interface signal 2, the quality is discriminated by an interface signal - NRZ signal conversion circuit (b), an error pulse detection circuit (c) and a quality discrimination circuit (d) to obtain quality discrimination resulting information 5. Then the input interface signals are being selected sequentially under the control of the series selection signal 8 to obtain the quality discrimination resulting information 5 corresponding to each series. After the quality discrimination resulting information 5 is stored sequentially in a discrimination result storage circuit (e), an overall discrimination circuit (f) outputs transmission line quality deterioration information 7 as the overall discrimination of the transmission quality of the n-series of the transmission lines for each period of the series selection signal 8. Thus, the scale of the circuit is reduced and the power consumption is decreased.