PURPOSE: To measure the insulation resistance of a line without being affected by the error of individual elements inside a measuring circuit.

CONSTITUTION: A voltage E from a reference power supply 1 is divided by voltage-dividing resistances 2, 3, 4 which are connected in series, divided voltages are changed over by a first changeover device 5A, they are applied to a line 7 to be measured, and the value Rx of a resistance 8 to be measured is found on the basis of a measuring voltage V which is obtained at this time. In addition, two reference resistances 6A, 6B are prepared. When the first changeover device 5A is changed over to the side of the reference resistances (6A, 6B), the divided voltages from the first changeover device 5A are applied respectively to the first reference resistance 6A or the second reference resistance 6B by using a second changeover device 5B. The resistance value Rx of the resistance 8 in the line 7 to be measured is corrected by two voltages V1, V2 obtained by the changeover of the second changeover device and by two reference resistance values r1, r2. Thereby, the resistance value of the resistance 8 can be computed precisely without being affected by the error of the elements constituting a measuring circuit.