PURPOSE: To suppress corrosion and stably operate an electric appliance with high reliability for a long period of time, by using a cooling medium such as flon recycling it through a heat exchanger and a decomposition product- removing member packed with a specific compound.

CONSTITUTION: The body 1 of a heat-generative electric appliance such as a transformer or a condenser is enclosed in a tank 3. The space 2 between the appliance and the tank is filled with an electrical insulating cooling medium such as a flon or a fluorocarbon to cool the body 1 while the medium is circulated with the aid of a pipe 4 and a circulating pump 7 through a heat exchanger 5 where the medium is cooled, and a decomposition product-removing member 6 packed with a hydrotalcite compound of the formula I (wherein x, y and z satisfy the relationships of the formulae II and III; and m is an integer) where a corrosive product etc., formed by decomposition of the medium is removed, thereby enabling the appliance 1 to be stably operated for a long period of time.