PURPOSE: To easily connect threads by putting threads to be connected oh a grooved metallic mold that their connecting parts may be overlapped, and putting a thermoplastic resin film on them and then heating it, and finally making edges of grooves on the metallic mold slit the film.

CONSTITUTION: Fiber threads a, b... and a', b'... are arranged in parallel to each other on a metallic mold having edges 3a, 3b.... A thermoplastic resin sheet 2 is put on the fiber threads, and a hot plate 4 is put on the resin sheet 2 to heat it for making the resin sheet 2 connect the threads (a) to (a'), and (b) to (b')... respectively, and then the resin sheet 2 is slit by the edges 3a, 3b.... Thus the ends of threads can be easily and surely connected.