PURPOSE: To achieve a higher reliability, by arranging a detection element for detecting any one of harmonic current, arc light and arc sound generated between a pair of electrodes and a automatic stopping circuit which is operated by an interruption signal to interrupt a primary coil current and a time counter.

CONSTITUTION: An arc generated on a sample insulating material 7 between a pair of electrodes 6 is detected with a detection element 10, the output signal of which is inputted into an interruption signal circuit 20. Then, with the interruption signal of the circuit 20, a relay 16 is operated to open the contact thereof and the excitation of a main electromagnetic switch 1 is cut off to stop the output of a leakage transformer 8 by opening of a main switching contact 9 while a contact RC 3a of the switch 1 is opened to stop the time counter 5, with which the arc resistance of the insulating material 7 is indicated. Thus, when an arc discharge shifts to breakdown current on the surface of the insulating material 7, the apparatus can be stopped automatically to eliminate artificial errors on the decision on the arc resistance time while saving labor for the measurement of the arc resistance time.