PURPOSE: To control a warp tension at an optimum level and to enable the weaving in high efficiency by setting the shift level of a back roller relative to the angle of the main shaft of a loom based on the warp tension controlling elements such as warp opening state.

CONSTITUTION: A dobby signal transmitted from a warp-opening driving apparatus 18 is inputted into an arithmetic unit 22 and the shift level of a back roller 14 is calculated according to a preparatorily stored shift level table map corresponding to the warp tension controlling elements such as the opening extent of warp, opening timing and opening pattern. The calculated result is transmitted to a setting device 23 and the obtained shift level setting signal is outputted to a shift controller 21. The controller 21 actuates a hydraulic cylinder 19 based on a preparatorily stored drive-controlling table of the cylinder corresponding to the above shift level setting signal and a separately inputted main-shaft angle signal. The back roller 14 is shifted by swinging a tension lever 24 to control the warp tension.