<p>A pair of false twist belts is entrained, respectively, about pulleys each subjected to crowning. Under such an arrangement as the outer surfaces of the belts intersect, one false twist belt is brought into pressure contact with the other false twist belt and a thread is passed between these false twist belts and twisted. Flexural rigidity of the false twist belt in the width direction is set to such a value as the outer surface of the false twist belt is curved convexly along the belt width direction at a position where the false twist belts come into pressure contact.</p>
