Abstract: In a jet loom in which weft yarns are stored in weft yarn storing units according to a weaving command, and are jetted in the weft insertion cycles thereof, a device for controlling the storing unit comprises: rotation detecting means for detecting the rotational condition of the loom spindle, to output a spindle rotation signal; a rotation control circuit which, according to a weft yarn jetting order, calculates the number of standby cycles between weft insertion cycles, and calculates the speeds of rotation of the storing units to store weft yarns in advance according to the numbers of standby cycle, to output rotation command signals; a drive circuit for receiving the spindle rotation signal and the rotation command signal, to output drives signal to determine the speeds of rotation of the storing units; and drive means provided for the storing unit, each receiving the drive signal to drive the storing unit at a speed of rotation corresponding to the drive signal thus received, a weft yarn as long as a pre