PURPOSE: To secure safety to leakages by arranging the constitution such that the power supply to an electric apparatus is broken when an earth cable is not connected.

CONSTITUTION: When an electric apparatus is connected to commercial power source, a polarity judging means 3 judges which is the grounding side A of the commercial power source between two input lines 6a and 6b, and according to that the contacts 6a and 6b of a relay is changed over, whereby the polarity of the electric apparatus 1 is conformed to that of the commercial power source. In a transformer 8 being a power source means for small power, low voltage at a level of not affecting the human body is induced, which is turned into DC voltage with a rectifier 9, and this is grounded by an earth cable 12 through resistors R1 and R2. If the earth is not taken, the input voltage to a comparator 10 becomes high, whereby the power supply to the coil 11 of a relay 13 is interrupted, and the relay 13 becomes off and the power supply to the electric apparatus 1 is interrupted.