PURPOSE: To detect surge without using a high performance voltage divider or amplifier by inserting a surge absorbing element such as an arrester between a circuit to be inspected and the voltage divider.

CONSTITUTION: The voltage supplied from a circuit 1 to be inspected through a surge absorbing element Ar is divided by the resistors R1, R2 of a voltage divider 2 to be sent to an amplifier 3. The relay contact K inserted in the resistor R1 in series is instantaneously opened after necessary processing is finished to stop the discharge of the surge absorbing element Ar. The condenser C connected to both terminals of the resistor R2 prevents the input of high voltage noise to the amplifier 3 of the next stage. By this method, in such a case that the element Ar such as an arrester is inserted between the circuit 1 and the voltage divider 2 and the surge equal to or more than the operation voltage of the element Ar is superposed on the output voltage of the circuit 1, the element Ar holds an output state showing that surge is superposed even after the surge disappears. Therefore, surge can be detected without using a special resistor or ultrahigh speed amplifier.