PURPOSE: To execute the crosslinking of a thick insulator without reducing the dielectric strength by exposing crystalline polyethylene which is the insulator to a radiation in the fused state.

CONSTITUTION: An insulator mainly consisting of a crystalline polyolefin such as a polyethylene or ethylene copolymer having a crystallinity calculated by density of 50% or more is held at a temperature higher than the melting point of the crystalline polyolefin. The insulator is exposed to a radiation such as X-ray or γ-ray in this state to manufacture a cross-linked plastic insulated power cable. Thus, a reduction in dielectric strength which is caused in the radiation exposure at ordinary temperature can be solved.