To prevent the damage of a linear body wound around a delivery bobbin even if the traveling linear body such as an optical fiber is broken by forcibly stopping the bobbin, both driving wheels and all wheels constituting screening mechanism when the wire breaking of the linear body is detected.

When the position fluctuation of a dancer wheel 8 is detected to detect the breaking of a traveling optical fiber 20 by a wire breaking detector, a driving device 21 provided at each wheel receives its signal and forcibly stops the rotation of each wheel. Since control devices are thus provided at all bobbins and wheels, even if the linear body 20 such as an optical fiber, yarn, extra fine metallic wire or enameled wire is broken during a screening test, only the broken end of the linear body 20 is moved by its acceleration so as to prevent the linear body (product) wound around a delivery bobbin, from being damaged due to the end being wound onto the wheel to impart excessive tension to the linear body.