PURPOSE: To obtain the title apparatus having a yarn-separating device movably supporting a separator along running locus of yarn between clamp cutter rotatably provided centering yarn axis at both ends of yarn treating region and clamp and capable of giving strong and non-thick seam.

CONSTITUTION: A yarn separation device movably supporting a pair of separators 3 capable of separating untwisted double yarn 1 into two single yarns 1a and 1b along running locus of yarn between clamp cutter 2 for clamping yarn 1 to be treated consisting of twisted double yarn and uniforming yarn end and clamp 4 for demarcating separation section of double yarn 1 so as to be able to advance and retreat from the both sides to the yarn 1 to be treated is provided and either one of clamp cutter 2 and clamp 4 is supported rotatably centering yarn axis of yarn 1 to be treated to constitute untwist separation apparatus of double yarn.