A measurement method is disclosed for measuring the resistance component leakage current. The signal waveform of at least one AC cycle is sampled in the resistance component leakage current measurement, and the resistance component leakage current is measured. A zero-cross detecting circuit for detecting a voltage zero-crossing point of the target measurement circuit and outputting a sampling start signal is provided, voltage digital data of the sine wave form whose effective value is 1 and the output signal of the zero-phase current transformer are sampled, and the products of multiplication are averaged by synchronizing the sampled output signal value and the voltage digital data to obtain an active leakage component current of leakage currents.
