PURPOSE: To prevent the breakage of a circuit to be protected by a method wherein a power supply line for the circuit to be protected is cut automatically when the circuit is connected to an AC power supply having a high voltage higher than a predetermined voltage.

CONSTITUTION: When the input terminals 1a, 1b of a rectifier 1 are connected to an AC power supply, whose voltage exceeds a predetermined level, the DC voltage between output terminals 1c, 1d exceeds a predetermined voltage whereby a first switching transistor 3 is put ON and an electric current, being conducted through a second switching transistor 8, flows through the first switching transistor 3. As a result, the second switching transistor 8 is put OFF and the exciting of a circuit 100 to be protected is stopped. According to this method, the breakage of the circuit 100 to be protected can be prevented.