PURPOSE: To prevent cutting, damage or fluffing of fragile fiber bundle by detecting tensile force of a fragile fiber bundle provided with loosening liquid and fed to a doubler through a dancer tensile detector and controlling a bobbin driving means.

CONSTITUTION: A plurality of bobbins 4 in a bobbin stand 6 are fed with loosening liquid from a loosening liquid feeding means 20 in respective pans 22, then fed out through a bobbin motor M1 with a fragile fiber bundle, i.e., a carbon fiber bundle yarn 2, being loosened, then tensile force thereof is detected through a tensile force detector 16 comprising a dancer 74 and a dancer roller 76, thereafter the yarns are doubled through a doubler 8 and wound around a bobbin 12 of a winder 14 through driving of a winding bobbin motor M2. Rotation of bobbin motors M1, M2 of the bobbin stand 6 and the winder 14 is controlled corresponding to the tensile force detected through the tensile force detector 16. Consequently, cutting of the fragile fiber bundle, damage on the surface of the fiber bundle and fluffing can be prevented.