PURPOSE: To prevent a reduction in the strength of a connection by forming separate twisting sections along the length direction of a connecting section reducing the strength of the connection while preventing the ends of two untwisted fiber portions from contributing to twisting, and thereby strengthening the yarn of this section.

CONSTITUTION: At a specified time, rotations in directions opposite clamps 4 and 5, i.e., in the directions F2 and F1, are started and, thus, an operation for gradually returning twisting in portions receiving excessive untwisting to zero is performed. Because of the rotations of these portions receiving excessive untwisting in the twisting directions, the free ends of untwisted fiber portions are helically rotated around the portions receiving excessive untwisting and, when the twisting of the excessive untwisted portions becomes zero, forward twisting is applied to the free ends of these portions. Then, plates 7 and 8 are opened, but the rotations of the clamps 4 and 5 in the twisting directions are continued, twisting is applied from both ends of a fiber section to a center part, and then twisting in this section is completely recovered.