PURPOSE: To efficiently impregnate oil by inserting by pressure a sponge bar impregnated with silicone oil in advance into the center hole of a stress cone to be impregnated with oil and impregnating lubricating oil on the inner face of the stress cone.

CONSTITUTION: A columnar or cylindrical sponge bar 11 made of sponge rubber is inserted into a stress cone 10 to be impregnated with oil while the inner diameter of its center hole 10a is made smaller than the diameter of the sponge bar 11. This sponge bar 11 is heated and impregnated with silicone oil in advance, this bar 11 is inserted by pressure into the center hole of the stress cone 10 and left for a while. The sponge bar 11 is elastically deformed by the pressing pressure due to the diameter difference between the outer diameter of the sponge bar 11 and the center hole 10a of the stress cone 10 to impregnate silicone oil. Silicone oil is simply and efficiently impregnated on the inner face of the stress cone 10 at the fitting site of the stress cone 10, thereby the stress cone 10 is made easy to be inserted with a cable, and the electric characteristic can be improved.