PURPOSE: To prevent a defective product occurring in a final inspecting step and to improve productivity by dipping a compound foil in electrolyte, continuously applying an AC or DC voltage and evaluating a time of a temperature rise of the electrolyte in comparison with a value of a previously measured product capacitor.

CONSTITUTION: Two compound foils 10 as materials under test are opposed so as not to be brought into contact with one another, a thermocouple 16 is inserted between the two foils 10, and dipped in a vessel 12 containing electrolyte 14 of ethylene glycol solution which contains ammonium borate. Then, an AC voltage equal to a rated voltage of a capacitor is applied to the foils 10, a time required to vary a temperature of the electrolyte 14 from 40 to 50°C is measured, and 15min is obtained, The same foil 10 as the material under test is used, a starting aluminum electrolytic capacitor is trially formed, a rated voltage is applied, a safety unit operating time is measured, and 19min is obtained. Since the safety unit operating time of a product allowance standard of this case is 15min, if it required 15min or more, it is allowed.