PURPOSE: To improve the mechanical characteristics such as the repeated bendability by coating bundled fiber cores with a plastic composition contg. a blowing agent which is gasified by heating, placing loosely a flexible metallic pipe aound the coated fiber cores, and carrying out heat treatment.

CONSTITUTION: Bundled fiber cores are coated with a plastic composition consisting essentially of vinyl chloride and a blowing agent which is gasified by heating such as azodicarbonamide or microcapsules contg. an org. liq. with a low b.p. A flexible metallic pipe such as a corrugated stainless steel pipe is loosely place around the coated fiber cores, and the plastic composition is expanded by heating to fill the space between the metallic pipe and the fiber cores. Thus, the mechanical characteristics such as the repeated bendability are improved.