PURPOSE: To increase the resolution, by binary value discrimination with two kinds of threshold values in which the level is automatically changed according to analog video signal.

CONSTITUTION: When an analog video signal (a) rises from the minimum value of the waveform, a capacitor is finished for the discharge at a time point, and when the signal (a) rises further, it passes through the threshold value V in which the voltage at the end of discharge of the capacitor 5 is added with the leading forward voltage of a diode 3, the diode 3 is conductive, and the capacitor 5 is charged with the signal (a). Next, when the signal (a) passes through the maximum value, starts trailing and passes through the threshold value W where the trailing forward voltage of a diode 4 is subtracted from the voltage at the end of discharge of the capacitor 5, the diode 4 is conductive and the voltage of the capacitor 5 is fallen down in following to the signal (a). Thus, the discrimination of binary value is made by two kinds of threshold values in which the level is automatically changed according to the input signal, allowing to increase the resolution.