A weft yarn storage unit disposed between a yarn supply package and a weft insertion device for storing a weft yarn to be inserted in the warp shed, comprises a differential yarn feeding device for feeding the weft yarn from the yarn supply package, and a yarn storage means for winding the fed weft yarn therearound by co-operating with the differential yarn feeding device, storing the wound weft yarn therearound and unwinding the stored weft yarn therefrom by co-operating with the weft insertion device, the yarn storage means being continuously rotating in a direction opposite to the direction in which the weft yarn is unwound at a predetermined circumferential speed in response to an insertion speed of the weft yarn. This weft yarn storage unit allows high-speed weft insertion while preventing the yarn breakage by reducing ballooning of the weft yarn when the weft yarn is released off the yarn storage drum.