To measure the winding tension of a yarn before doubling in a yarn doubler.

This doubler comprises at least a winding device R and a first yarn guide 31 having at least a feed roller 20b installed at a position corresponding to the winding device R and a feed roller 20a installed at the other position, and further comprises a first yarn guide 31 installed in proximity to a yarn path for a yarn Yb fed from the feed roller 20b, and guiding a yarn path for a yarn Ya fed from the feed roller 20a along the yarn path for the yarn Yb fed from the feed roller 20b. The first yarn guide 31 is installed separably in the direction separating from the feed roller 20b.