To prevent the weft breakage caused by the slackening of a warp especially near the selvage at the initial stage of warp opening when the opening of warps is still in a narrow state.

An air cylinder 1 having a shell-formed head 2 at the tip end is placed on a raceway 4 of a loom at the reverse side of a reed 5 relative to the cloth fell and near the outer end of the warp of the warp feeding side in such a manner as to enable the free insertion and extraction of the shell- formed head 2 into the warp opening. The shell-formed head 2 is inserted into the opening immediately before the insertion of weft with a main nozzle 8 and extracted immediately before the closure of the warps.