A voltage level of an output node is reversed by an inverter. An output of the inverter is used as an initial setting signal. A feedback switching transistor is also provided in an initial value setting circuit. Furthermore, there is provided a limiting device for eliminating an abnormal operation of the feedback switching transistor until the inverter starts a normal operation, in the event of momentary power failure or when the electric power source is quickly turned on after a very short break. Accordingly, normal operation of the inverter is assured even in such an abnormal condition accompanying momentary interruption of power supply, thereby surely outputting an initial setting signal.