PURPOSE: To eliminate with high efficiency the peroxide contained in the oil which is caused in the process of deterioration of the oxidation of the insulating oil, by setting a container filled with the active alumina and silica gels at a flow path of an electric insulating fluid like an electrical apparatus, etc.

CONSTITUTION: The electric insulating oil is put into a tank 6 containing a filler of the active alumina and silica gels from an electrical apparatus like a large-size transformer 1, etc., and oil cleaner, etc. and through a circulating pipe 4. The insulating oil is stirred in the tank 6 and passes through a filter 9 to prevent the filler from flowing into the transformer 1, etc. and is then sent back to the transformer 1 through a circulating pipe 5. During this circulating process, the peroxide in the insulating oil caused by the deterioration by oxidtion can be selectively eliminated.