A Schmitt trigger circuit comprising input and output terminals, first and second voltage supply lines, a first transistor having its base connected to the input terminal and its the collector connected to the first voltage supply line though a load resistor, a second transistor having its collector connected to the first voltage supply line, a first constant-current source through which the first and second transistors have their respective emitters commonly connected to the second supply voltage line, and a third transistor having its collector connected to the first voltage supply line, its base connected to the first voltage supply line through the load resistor and to the collector of the first transistor and its emitter connected to the base of the second transistor. There is further provided a second constant-current source through which the third transistor has its base further connected to the second voltage supply line.