PURPOSE: To prevent unexpected troubles from occurring due to bad grounding by monitoring the grounding condition of an electric apparatus at all times and automatically stopping supply of power to the electric apparatus when a bad grounding is recognized.

CONSTITUTION: Two power lines 3a, 3b of a commercial ac power supply which are wired in an electric apparatus are connected to a grounding terminal 7 for connecting a grounding line of the electric apparatus through small capacity capacitors C1, C2 respectively. A current sensor 6 is installed to detect leakage current which flows to the grounding terminal 7 through the power lines 3a, 3b. And, an amplifier 8 to amplify a signal of the current sensor 6 is installed. Furthermore, the power lines 3a, 3b are provided with normally-closed relay contacts 13a, 13b to cut off the supply of power to a load 1. When the leakage current is a specified level or lower, the normally-closed relay contacts 13a, 13b are opened.