A precursor fiber bundle for manufacture of carbon fibers, comprising a first precursor fiber bundle for manufacture of carbon fibers, said first precursor fiber bundle having 30,000 or more filaments and having a flatly opened undivided terminal end wherein said terminal end is the only portion of said first precursor fiber bundle flatly opened, and a second precursor fiber bundle for manufacture of carbon fibers, said second precursor fiber bundle having 30,000 or more filaments and having a flatly opened undivided starting end wherein said starting end is the only portion of said second precursor fiber bundle flatly opened, wherein the terminal end of said first precursor fiber bundle and the starting end of said second precursor fiber bundle are directly joined, and at a joined portion where the terminal end of said first precursor fiber bundle and the starting end of said second precursor fiber bundle are joined, the individual filaments of said undivided ends of said respective precursor fiber bundles are substantially uniformly interlaced with each other and said interlacing of filaments is in a pattern selected from the group consisting of continuous substantially uniform interlacing in the transverse direction of said fiber bundles, interlacing at many joints on the joining portion, and interlacing of the entire face of said joining portion.