Switch box means

Discharge location determination unit (20) comprises a connecting metal electrode (5a, 5b) respectively and each input of the discharge accompanying the discharge detection circuit (10a, 10b) signal, and is input to the discharge detection circuit (10a, 10b) of the output signal the arithmetic circuit (11), an arithmetic circuit (11) the difference in intensity of the two output signals or intensity ratio is within a predetermined value in the case, it is determined that the discharge location in the tank (1) interior, a case where the outside thereof, is determined to be outside of the discharge tank (1).