To provide a polymer insulating tube with a capping group wit different leakage distances, long enough to economically obtain leakage distances required for a preset staining degree, and a mold used in manufacturing it.

A polymer insulating tube 1 has a core member 2 and a sheath 3 made of insulating polymeric material and formed on the outer periphery of the core member 2, the sheath 3 consisting of the sheathing part 4 to cover the outer periphery of the core member 2 and capping parts 5 provided at proper spaces in the longitudinal direction to be extended from the outer periphery of the sheathing part 4 to the outside in the radial direction. If L is a leakage distance between the capping parts 5 and H is the overhang of the capping parts 5 from the sheathing part 4, a capping group different in L/H is partly arranged over the longitudinal direction of the polymer insulating tube 1. A mold used in manufacturing it has a cavity to form caps in the capping group different in L/H over the longitudinal direction.