To provide high strength and high durability insulating cylinder, with which the middle joint and end treatment of a power cable can be practiced satisfactorily and a power cable joint with excellent electrical performance can be formed.

An insulating cylinder 10, having an expanded diameter supporting cylinder comprises a normal temperature shrinkable insulating cylinder 12 and an expanded diameter supporting cylinder 14, by which the normal temperature shrinkable insulating cylinder 12 is supported in an expanded diameter state. The expanded diameter support cylinder 14 is composed of two layers of inner and outer cylinderical cores 14A and 14B. Both the cores 14A and 14B are composed of spiral cores 16 and 17 consisting of string-shaped elements 16a and 17a which are made of plastic such as polypropylene, etc., and are wound into spirals in order to facilitate decomposition.