To completely remove a die splitting line of an insulator, to improve adhesion of silicon rubber, and to eliminate void by dipping its bottom half circle part in silicone rubber solution agent, while horizontally rotating the insulator having the die splitting line coated with the silicone rubber.

A glass fiber reinforced plastic pipe whose surface is coated with primer is arranged in a mold liquid silicone rubber is injected in a cavity and solidified so that a silicone rubber insulator coated with silicone rubber and formed into a prescribed shape is provided. While the silicone rubber insulator 20 having a die splitting line formed in the surface is horizontally rotated in the molding, the approximately half circle part in the bottom of the horizontal line is immersed in silicone rubber solution agent 2 stored in a removal tank 3 in a removal device 11. This method cna effectively remove the parting line, uniformalize the surface, and improve flashover voltage. The silicone rubber solution agent 2 is sent to a distilling device 12 by a pump 4, and regenerated by the distilling tank 5 having a heater 8 to be reused.