PURPOSE:To obtain a stable zero crossing pulse as a comparison output, by stopping the discharge of an integral network for obtaining the temperature signal voltage, in the period for generating a zero crossing pulse for comparing the temperature signal voltage and the temperature setting voltage. CONSTITUTION:A transistor TR24 of a discharge stopping circuit 25 is turned on through a resistance 26 when a zero crossing pulse Vz is not generated from a zero crossing pulse generating circuit 17. On the other hand, the pulse Vz is generated, and in the period for comparing the temperature setting voltage Vs and the temperature signal voltage VT, the TR24 turns off, and stops discharge of a capacitor 13 of an integral network 15. Accordingly, in this period, voltage of the voltage VT is not varied, and even when the voltages Vs and VT stand close to each other, a period of VT>Vs is not generated in the latter half of a square waveform, and a phase control of a power control element by an output of V0 of a comparing circuit 16 is not executed. In this way, the voltage VT and Vs are compared exactly, and a stable zero crossing trigger pulse is obtained as a comparison output.