PURPOSE: To provide the title fiber constituted by carbonaceous fiber of specified diameter produced by vapor growth process with polyarylene sulfide portion bound and added thereto, having high dispersibility in and affinity for solvents, rubbers, resins such as polyphenylene sulfide resin.

CONSTITUTION: The objective fiber constituted by carbonaceous fiber 0.01-4 (pref. 0.02-1) μm in diameter produced by vapor growth process with polyarylene sulfide portion bound and added thereto. The binding and addition of the polyarylene sulfide portion to the carbonaceous fiber can be accomplished, for example, by the following process: firstly, halogen-bound-and added carbonaceous fiber is synthesized; and this fiber is then reacted by heating with sodium sulfide in combination with a dihaloaromatic compound in a polar solvent.