PURPOSE: To eliminate irregularities in twisting and make it possible to maintain a sufficient strength in a tying process in a spinning of fiber bundle uniting, by effecting a swirled flow having a rotation and advancing direction reverse to those of a first swirled flow to the end of a broken thread on the winding side upon the detection of thread breakage.

CONSTITUTION: A thread breakage detecting lever detects a thread breakage when it occurs, and therefore a first swirled flow having a component speeds of fiber bundle advancing direction and rotating direction, from a twisting jet nozzle 17, ceases, but a second swirled flow in the direction reverse to that of the first swirled flow, from a tying jet nozzle 20 and a third swirled flow from a fiber bundle propelling jet nozzle 24 generate so that fiber bundles introduced into a twisting passage 6 from a throttle part 12, are sucked out through an intake air hole 22. The ends of fibers which are pulled out from cheeses and set in a passage 15, are sucked into a suction hole 22 under the action of the second swirled flow so that they whirl in opposite directions, respectively and twined together thereby they are tied together. After that, the formation of a yarn is continued. Therefore, there is no untwisted part in the tied part so that the strength yarn may be maintained even in the tied part.