To distinguish occurrence of internal short circuit from other external short circuits by detecting the pressure value of an insulation medium, integrat ing its pressure values for each prescribed section for computation of a plurality of integrated values, and determining the generation of the internal short circuit from the respective integrated values.

A pressure detection part 14 detects pressure in a container 11, a pressure sampling part 15 converts the detected pressure into a prescribed signal, and a computing means 17 integrates the converted pressure values for each prescribed section and computes a plurality of continuous integrated values. When a value exceeding a prescribed limit threshold is detected, it is determined that generation of an internal short circuit in an insulation medium electrical apparatus is present, a signal is transmitted to a bursting plate destructing means 19 to destroy a bursting plate 13. A recording means 20 records this result. Accordingly, generation of internal short circuit can be distinguished from other external short circuits.