Abstract: A Schmitt trigger circuit has a first transistor for inversion having a base supplied with an input voltage. A level shift diode shifts an emitter voltage of the first transistor from a reference level, and a second transistor for switching has a collector and an emitter respectively coupled to an anode and a cathode of the level shift diode. The emitter of the second transistor is coupled to a first power source voltage. A switching part is coupled between a base of the second transistor and a second power source voltage higher than the first power source voltage, for controlling switching of the second transistor. An output circuit is coupled between the first and second power source voltages and supplied with the input voltage through the first transistor for outputting the output voltage. The switching part is controlled responsive to a voltage from the output circuit so as to close when the logic level of this voltage is the high level, thereby turning the second transistor ON.