Abstract: Details are presented of the design features of LS type micro leakage Cu-CuSO₄ electrode as being reference electrode for comparing underground structure protection potential. Comparisons are made of this electrode having various filler length with ceramic film electrode under water soak test. Test results showed that the leakage rate of CuSO₄ of former is lower by three magnitudes as compared with ceramic film electrode and conventinal saturated CuSO₄ electrode. The life expectancy is estimated in two years.