PURPOSE: To prevent an unstable state of a threshold value of an output in the case there is a fluctuation in a crest value of an input signal by switching the output threshold value in accordance with an output of a hysteresis generating means.

CONSTITUTION: When a state that a crest value of an input signal is smaller than a reference value is varied to a state that the former is larger, an output for showing its variation is inputted from a level discriminating means 12 to a hysteresis generating means 13. Subsequently, a signal is outputted to a threshold value switching means 14 after some delay time, for instance, after several clocks, after a signal from the level discriminating means 12 is inputted. At such a time point, a threshold value outputted from the threshold value switching means 14 is switched. Also, when a state that a level of an input signal is larger than a reference value, is varied to a state the former is smaller, as well, switching of the threshold value is executed after several clocks elapse after its variation in the same way. In such a way, even if the case there is a fluctuation in the crest value of the input signal, an unstable state of a threshold value of an output can be prevented.