PURPOSE: To enable a high-frequency signal to be used as a signal for measurement by providing a current transformer, an oscillator, and an adder.

CONSTITUTION: Oscillators OSC1 and OSC2 of frequencies f1 and f2 (f1>f2) are added to the primary winding of an injection transformer OT to apply the signal for measurement between a cable run and the ground. Therefore, a leak current which is fed back to an earth line LE is detected by the current transformer ZCT, whose output is amplified by an amplifier AMP and inputted to a filter F1 for detecting a component of frequency f1 and a filter F2 for detecting a component of frequency f2. Then the output of the filter F1 is inputted to one input terminal of a modulator M1 and the output of the oscillator OSC2 is inputted to the other input terminal; and the output of the filter F2 is inputted to one input terminal of the modulator M2 and the output of the oscillator OSC1 is inputted to the other input terminal. Thus, the outputs of the modulators M1 and M2 are obtained and applied to the adder ADD. Then the output of the adder ADD is applied to an LPF, whose output is applied to a modulator MULT to measure insulation resistance.