Abstract: A protection circuit for a switching mode power supply circuit which outputs a constant voltage interrupts the ON-OFF switching operation of the switching transistor when an overvoltage state is detected. This protection circuit includes a switching transistor, a diode, a phototransistor, a thyristor, and a capacitor, in conjunction with a photodiode, a zener diode and a thyristor. If either an overvoltage is detected or an overcurrent is detected due to the load circuit being short circuited, the protection circuit simultaneously prevent the power supply input from the power input line from being unnecessarily lost.