Several ways for monitoring the soundness of ZnO lightning arresters have been suggested, and all of which are based on the measurement of leakage current since it is well known that the resistive leakage current is a main indicator of arrester deterioration. In this paper, we proposed an algorithm to measure the resistive leakage current, which is quite different from the conventional method that eliminates capacitive current from the total leakage current. The proposed algorithm is based on that the magnitudes of the resistive leakage current are equal at the same applied voltage levels. To confirm the reliability of the algorithm, we fabricated a leakage current detector and designed an analysis program. Experimental results showed that the method does not need a complex circuitry and is useful to analyze the resistive leakage current.