PURPOSE: To make an accurate decision on defective insulation of a cable to be measured, by judging on whether DC voltage is generated in the cable being measured from four measured values of a voltmeter to allow effective measurement of an insulation resistance value thereof with an exact accuracy.

CONSTITUTION: A breaker 2 is opened and with a switch 11 closed, a selector switch 5 is turned to the changerover position (I) to select a specified resistance value. Under such a condition, the switch 11 is opened and DC voltage generated between a cable shield 5 and the ground is measured with a voltmeter 18. Then, the selector switch 15 is turned to the changeover position (II) to select a specified resistance value and the DC voltage is read with the voltmeter 18. Then, the selector switch 15 is returned to the changeover position (I), the switch 11 is closed and subsequently, the breaker 2 is closed while the cable 4 being measured is set under the AC service voltage. Again, the switch 11 is opened and a specified resistance is selected with the selector switch 15 to obtain the DC voltage by reading on the voltmeter 18. Then, the selector switch 15 is turned to the changeover position (II) to select a specified resistance and the DC voltage is obtained by reading on the voltmeter 18.