PURPOSE: To provide a complete logarithmic scale on a scale of a megger by flowing measuring and reference currents through a plurality of diodes to generate terminal voltages and processing the differential voltage of the terminal voltages.

CONSTITUTION: Rated voltages 1000, 500, 250, 100 volts or the like are used for rated voltage power supply terminals 1, 1'. An article to be measured is connected to measuring terminals 2, 2'. An insulation resistance value Rx provides a voltage at the terminal A by flowing the measuring current Ix flowing through the article to be measured at rated voltage through the first logarithmic converter diode D1. Rated voltage is applied to the reference resistor Rr to flow the reference current Ir flowing through the resistor Rr through the second logarithmic converter diode D2 to provide a voltage at the terminal E. Then, the differential voltage between the terminals A and E is amplified through high input impedance amplifiers Q1, Q2 to compensate the errors caused by the internal resistances of the diodes D1 and D2, and thus applied to a DC voltmeter M to indicate the voltage on the voltmeter M. Thus, the scale of the voltmeter may be completely converted to logarithmic scale.