A PNPN structure thyristor and a third P-type impurity region are formed on a semiconductor substrate, and a fixed current circuit using an NPN transistor 20 and a resistor 30 is connected to a cathode terminal K and a gate terminal G of a composite element 10 that has a P-channel MOS transistor Q3 with the third P-type impurity region as a drain connected to a PNP transistor Q1 of the thyristor. If a voltage applied to the anode terminal A of the composite element 10 rises from 0V, initially the thyristor is on and current flows, but once the threshold voltage of transistor Q3 is reached transistor Q3 turns on. A short circuit then exists between the base and emitter of transistor Q3 of the thyristor and current of the composite element 10 itself is cut off. Because of the bipolar structure it is simple to configure a small integrated circuit with a high withstand voltage. In this way, a zero crossing detection circuit is provided that has high withstand voltage characteristics, but which can be made as a small sized integrated circuit.