PURPOSE: To obtain the titled fibers having a strength capable of sufficiently withstanding practical use in spite of light weight, by adjusting the shearing rate of molten carbonaceous pitch passing through a nozzle to a specific value or more in extruding and spinning the pitch.

CONSTITUTION: Carbonaceous pitch e.g. molten coal tar pitch or naphtha tar pitch, etc., is extruded through a nozzle having <10 ratio of the nozzle length to the caliber at ≥400/sec, preferably 500W1,000/sec shearing rate to afford carbonaceous fibers, which are then infusibilized and further carbonized to give the aimed carbonaceous fibers.