A Schmitt trigger circuit according to an embodiment includes a voltage dividing circuit that divides an input voltage and outputs a divided voltage, and a basic Schmitt trigger circuit that includes a transistor as a current controlling element and controls current flowing through a light emitting diode (LED) included in an external photocoupler on the basis of the output voltage of the voltage dividing circuit proportional to the input voltage. The voltage dividing circuit has a positive temperature coefficient.