A yarn supply system for a loom includes a yarn supply member changing apparatus for replacing a using yarn supply member with a new one when the weft yarn of the used yarn supply member has been used up or broken. The new yarn supply member is supplied to the yarn supply member changing apparatus at a mounting position from a self-propelled carriage. The new yarn supply member is carried to a yarn supply position by a carrying member movably mounted on the yarn supply member changing apparatus. A weft yarn wound on a bobbin forming part of the yarn supply member has a free tip end section which has been inserted into the central hole of the bobbin. The weft yarn tip end section of the yarn supply member at the yarn supply position of the yarn supply member changing position is drawn out of the bobbin and threaded into a weft winding arm forming part of a weft measuring and storing device under the influence of air stream. Thus, the weft yarn is supplied through the measuring and storing device to a weft inserting nozzle which projects the weft yarn into a warp yarn shed so as to accomplish a weft picking.