In a process for producing a highly expanded polyolefin insulated coaxial cable comprising the steps of feeding a polyolefin, a nucleating agent and a volatile liquid as a blowing agent to an extruder and heat extruding said mixture through said extruder onto an inner conductor passing through said extruder to cover the outer periphery of said inner conductor thereby forming an insulation layer of a highly expanded polyolefin foam having a foaming ratio in excess of 2.5 times (60% expansion degree), the improvement which includes the step of, after extruding the foaming polyolefin mixture around said inner conductor, sizing the outer surface of said foaming polyolefin layer on said inner conductor by means of a sizing die to provide a highly expanded uniform polyolefin insulation layer of a predetermined size, said insulation layer being tightly adhered to said inner conductor.
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