To obtain carbon fiber of high quality without breakage at the jointed part or burning breakage by connecting precursor fiber bundles at their edges by needle-punching through a not-exothermic jointing medium at their flame- resisting temperature and then by firing the connected precursor fiber bundle.

Two of precursor fiber bundles with a filament count of 3,000 or more each are connected at their edge parts by using flame-resisting threads at the flame-resisting treatment (preoxidation) temperature as a non-exothermic jointing medium to flatly open the edge fibers of the crimped precursor fiber bundles into less than 400 filament count/mm so that the filament count F of the flame-resisting yarns and the filament count F of the precursor filament bundle may satisfy the formula: 0.4×G≤F≤1.5 and their edge parts of the precursor fiber bundles are heat-treated to remove their crimps. Then, they are overlapped, needle-punched and a flame-resisting reaction inhibitor as aqueous boric acid is applied to the jointed part and the jointed fiber bundle are fired to give the objective carbon fibers.