In recent years, the use of electric power has been rapidly increasing. Therefore, maintenance of distribution lines is important. Accidents of the distribution system are mainly earth faults. Consequently, an earth fault detection technique is required. On the other hand, due to the development of power electronics, distortion of the low voltage level power line occurs. In the Ior method, which is currently the mainstream method for earth fault detection, the earth fault current is calculated by a vector computation method in order to eliminate the earth capacitance of the line. However, in an actual distribution system, distortion has occurred in the line; therefore, we the current Ior method causes a large error in calculating the effective earth fault current.