PURPOSE: To produce a tank of an oil-immersed electric apparatus in small size having a light weight and to prevent the deformation by a method wherein the oil-immersing electric apparatus is put in a pressure reducing tank, the pressure in the pressure reducing tank is reduced to be lower than the atmospheric pressure, the inside of the tank of oil-immersing electric apparatus is made to be vacuum and an insulating oil is poured into the apparatus.

CONSTITUTION: An oil-immersing electric apparatus 7 like transformer, reactor, etc., is put in a pressure reducing tank 18 for oiling, and the pressure in the tank 18 is kept to be lower than the atmospheric pressure (for example, 0.8...0.5kg/cm2). A tank 7b of the oil-immersing electric apparatus 7 is made to be vacuum, and an insulating oil is poured into the tank from an insulating oil enclosing valve 8. By reducing the pressure applying to the tank 7b of oil-immersing electric apparatus like this, the tank 7b of oil-immersing electric apparatus can be produced in small size having light weight, and the curtailment of material can be realized and the deformation can be prevented.