A thread supplying device includes a rotator feeding a thread downstream of a thread supply path, the rotator winding a periphery thereof the thread several times and locating in the path extending from a thread supply part to a needle and loopers, a stepping motor continuously drivingly rotating in one way the rotator in synchronism with the main shaft of a sewing machine, thread feed setting means setting a unit thread feeding amount necessary for formation of seams per stitch, and control means automatically controlling the rotation of the motor to feed the unit thread feeding amount set by the setting means for each formation of seams per stitch. This device can stably supply a predetermined amount of the unit thread, and achieve stabilization of stitch performance, irrespective of the type of the thread, and whether the feed of a fabric increases or decreases by a rapid change in the rotational speed of the sewing machine.