In this paper, is suggested method of discriminate whether inferiority or not. The existing guarding method of incircuit tester was taken the unknown element value and wires were broken down in most of circuit. but the unknown element value of parallel LC circuit is difficult to measure. We infer characteristic of elements from spectrum of parallel connection, therefore, supply spectrum for guarding point and can discriminate inferiority of elements. In this way, node electric potential method be improved guarding method. In view of the result so far achieved, it is conformed that error rate is improved about 7%.