To make no misconnection to a power supply plug outlet, to enables invariable monitoring, and further to obtain a commercial power source from an internal power source.

This monitor is equipped with a 3-pin type power plug 21 adapted to a 3-pin type power outlet 10 of a on-side ground wiring system, a measuring instrument body 22 is has a ground fault measuring circuit 23, including a resistance 231 for voltage detection connected between a neutral terminal N and ground terminal E, while the neutral terminal N of its power plug 21 is considered as a circuit common terminal, and between the neutral terminal N and a power terminal L, a power circuit 24 for internal power supply is formed which includes a diode 243 which deters voltages from being applied from a half-wave rectifying circuit 242 and the power terminal L to a ground fault measuring circuit 2243.