PURPOSE: To detect faulty contact of an electric apparatus by detecting and integrating frequencies higher than specified of a power transmission system.

CONSTITUTION: A faulty contact detection circuit 1 comprises a current detection circuit 2, an integrating circuit 6, and an output latch circuit 11. A power transmission system is connected to the primary winding of a transformer 3. The frequency characteristic of the transformer 3 is adapted to frequency components generated by faulty contact. A rectification-integration circuit 4 rectifies the change in a current caused by faulty contact into a voltage waveform about 50μs-2ms is width and integrates it. A differentiation circuit 5 converts only the amount of change in voltage into a voltage signal of an appropriate pulse width. The integrating circuit 6 judges whether the change in the current of the power transmission system is due to faulty contact or is free from danger caused by intrusion of external noises and transmits the result through the output latch circuit 11.