An air guide member has therein an air guide opening and a slit through which the weft thread passes out of the opening. The opening has an inwardly projected land portion with first and second peripheral sides and includes an air induction section directly connected to the slit and a weft guiding section connected through the air induction section to the slit. The first peripheral side bounds partially the air induction section, while, the second peripheral side bounds partially the weft guiding section. An auxiliary nozzle is associated with one of the air guide members and has an air jet opening exposed to the air induction section so that the air jet from the opening advances toward the weft guiding section after passing through the air induction section. The opening is inclined toward the weft picking direction by a given angle with respect to the direction. An imaginary plane containing the second peripheral side of the land portion intersects the perimeter of the weft guiding section at a position away from the slit.