To cut off power supply if a ground terminal is not grounded.

If power supply is applied to a line between terminals 14, 15 with the voltages induced on both the sides of the neutral position 17 of a secondary winding 16 set so as to be a little different from each other and a ground terminal 18 ungrounded, a voltage detecting means 21 detects voltage occurring between the neutral position 17 of the secondary winding 16 and a non-hot-line terminal 15. A transistor 28 is conducted by the detected output, the luminous element 33L of a photocoupler emits light, a photodetector 33P is conducted to operate a relay 37, the contact switch 13 thereof is turned to NO on the normally-opened side, power supply to a primary winding 12 is cut off, and the condition is held in the self-holding circuit of the relay conducting NO on the normally-opened side of the switch 13. Voltage is not detected by the voltage detecting means 21 in such a state that the ground terminal 18 is grounded, so that the transistor 28 remains non-conductive.