PURPOSE: To prevent an inner conductive rubber layer from sliding and deforming due to the pressure of injection molding and to prepare a stable rubber product, by preparing an emboss pattern on the outer surface of a core metal where the inner conductive rubber layer is positioned.

CONSTITUTION: A premolded inner conductive rubber layer 1 is fitted onto a core metal 3 and the work is set in mold 5. An emboss pattern is prepared on the outer surface of the core metal 3 where the inner conductive rubber layer 1 is positioned to prevent the inner conductive rubber layer 1 from sliding and deforming due to the pressure of injection molding. An insulating rubber 2 is injected into the mold 5 from a rubber pouring hold 6 in this state. A plug-in type insulating pipe to be used for connecting part of electric cables is prepared by vulcanizing and molding it integrally.