PURPOSE: To obtain the titled carbon fiber of high strength with little fluff, by oxidizing a copolymer fiber having an acidic terminal group substituted by zinc while contrating the fiber with multistage roller under specific conditions, and carbonizing the resultant fiber to shorten the flameproofing treatment step.

CONSTITUTION: A copolymer containing a vinyl monomer having an acidic group, e.g. sodium methallylsulfonate, and at least 95% acrylonitrile is dissolved in a 58wt% aqueous solution of zinc chloride, and the resultant solution is extruded into a 25wt% aqueous solution of zinc chloride to give a fiber having a acidic groups wherein 50W100 equivalent % of the terminal acidic groups are substituted by zinc. The fiber thus obtained is then oxidized at 200W300°C with mutlistage roller groups. In the oxidation process, 20W50% the free shrinkage of the fiber is given thereto up to a combined oxygen content of 4W5%, and further oxidation is carried out while shrinking the fiber at a shrinkage of 50W70% the free shrinkage of the fiber. Carbonization is then carried out at 500W2,000°C in a nonoxidizing atmosphere while giving 40W70% the free shrinkage of the fiber thereto.