PURPOSE: To realize a Schmitt circuit with less effected by a noise component near a reference voltage in an input signal by providing a transistor(TR) having its threshold level near a VDD voltage and a TR having a threshold level near a GND voltage.

CONSTITUTION: When an input signal is descended and reaches a reference voltage REF 3 or below, the output of a comparator 1 changes form an H level to an L level. When the output of the comparator 1 reaches a threshold level (VTP) of an NMOS TR (PMOSTR) 6, the PMOSTR 6 is energized, the reference voltage is increased to REF 2 and when the output reaches the threshold level VTN or below, the PMOSTR 6 is energized, the reference voltage is increased to the REF 2, and when the output reaches the VTN further, the NMOSTR 7 is turned off and the reference voltage is increased to the REF 1. Thus, even in the presence of a noise component near the reference voltage in the input signal, the effect is made small.