PROBLEM TO BE SOLVED: To provide high-quality flame resistant fiber and carbon fiber at low cost by efficiently conducting a flame resistant treatment in a short time.SOLUTION: [1] polyacrylonitrile (PAN)-based flame resistant fiber is manufactured by modifying a PAN-based polymer by an oxidant containing a thiolate-based compound and a nitrogen atom in a solution and then spinning the modified polymer. [2] PAN-based flame resistant fiber is manufactured by modifying a PAN-based precursor fiber in a liquid phase containing an oxidant containing the thiolate-based compound and the nitrogen atom. The PAN-based flame resistant fiber obtained by the manufacturing method is heat treated at 300 to 3000°C to obtain carbon fiber excellent in mechanical strength. The thiolate-based compound is preferably a compound represented by the general formula (1), where Mis an alkali metal and Ris a hydrocarbon group or a hydrocarbon group having at least one of a hydroxyl group, an amino group, a nitro group, a thiol group, an imino group, a nitrile group or an azo group.
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