To provide a technology which can distinguish the generation of the disconnection of a B class earth for a transformation for receiving, or a high voltage/low voltage transformer for distributing a power.

A voltage to ground measuring circuit EVTC which measures each voltage to ground of an R phase, an S phase and a T phase with a voltmeter V is formed by electrically connecting the R phase, the S phase and the T phase of the secondary side of a three-phase three-wire type transformer and a B class earthing wire and electrically connecting the respective phase and the B class earching wire by operating a voltage change switch VS. The disconnection of the B class earthing wire is judged by detecting the change of the voltage to the ground of the S phase.