PURPOSE: To make it possible to monitor insulation resistance by converting zere-phase voltage and zero-phase current into installation resistance through an insulation resistance operation unit, and indicating the resultant insulation resistance on display units in the form of locus and value.

CONSTITUTION: Zero-phase current transformers (ZCT) 2 transform the zere- phase currents of motors into small currents, and the resultant small currents are fed, as insulation resistance calculation data, into a insulation resistance operation unit (AA) 7 through ground directional relays 3. Zero-phase voltage, or insulation resistance calculation data, is detected by a transformer 4 for instruments that reduces the voltage of a buns, and fed into AA 7. AA 7 calculates insulation resistance from the input zero-phase current and voltage. The calculated insulation resistance is sent to an analog display unit 8, a digital display unit 13, an alarm setting unit 9 and an variation operation unit 10. The analog display unit 8 sequentially plots insulation resistance and thus indicates it in the form of locus. The digital display unit 13 indicates the values of insulation resistance. The alarm setting unit 9 produces an alarm signal 12 if insulation resistance is reduced and goes beyond a set value.