PURPOSE: To always detect insulation abnormality without fetching a signal from a high voltage section, by comparing ground currents on both sides of an insulating connection part of a cable to judge abnormality in the insulation when either thereof increases relatively.

CONSTITUTION: Ground currents flow to leads 9A and 9B linked to an insulating connection part 3C as measuring point from cables 1C and 3D on both sides thereof and then, to first and second coils 12A and 12B respectively. An insulation abnormality detector 10 is adjusted with a tap changer 13 beforehand so that a magnetic flux in a circular core 11 is zero when a cable line 4 is normal. A charge current Ic of the ground currents depends on the electrostatic capacitance of the cables while the effective current IR thereof depends on the insulation resistance thereof. If a change is caused in the dielectric constant insulation resistance of one cable 1C or 1D, current flows to a third coil 13C and an alarm 15 sounds as it exceeds a specified level normal.