PURPOSE: To obtain a grounding detector which can positively detect generation of grounding at any part of a power distribution path system using a balanced type AC power supply.

CONSTITUTION: The title detector is provided at an AC electric path 3 connecting a star grounding type AC power supply 1 and an AC electric motor 2 and is provided with a zero-phase current transformer 7 for detecting grounding current, a magnitude circuit 15 for obtaining the magnitude of the grounding detection signal from the zero-phase current transformer 7, a level discrimination circuit 10 for discriminating the level and signal width of the output signal of the magnitude circuit 15, and a signal width discrimination circuit 11. Further, it is constituted of a trigger circuit 12 for generating a trigger signal based on the output signal of the signal width discrimination circuit 11, a switching element which continues by the trigger signal, and an electromagnetic device 13 which is energized by the continuity of the switching element 14 and cuts off a circuit breaker 4 which is provided at the AC electric path.