To provide a normal temperature-shrinkable rubber unit of high quality, which is available in short manufacturing and delivery time and at a low manufacturing cost, by shortening the time required for forming an external semiconducting layer because of ease of formation and preventing fluctuations in the thickness of the external semiconducting layer.

The rubber unit includes: an internal semiconducting layer 1, which is expanded and supported on a dismountable diameter expansion member and formed mainly by rubber materials, such as, ethylene propylene rubber (EPR) and silicone rubber (SR); a reinforced insulating layer 3; an external semiconducting layer 11, stress cone parts 7, 7 on both sides of the reinforced insulating layer 3; and separation parts 9, 9 which are provided near the stress cones 7, 7 of the external semiconducting layer 11 and separating the external semiconducting layer 11 in a single longitudinal direction. The rubber unit is formed into a single piece, by molding the internal semiconducting layer 1, the reinforced insulating layer 3, the external semiconducting layer 11, and stress cones 7, 7 nearly cylindrically.