A motor controller using an inverter (103), which is useful for preventing coasting at the time of power failure or for keeping synchronous operation of a plurality of motors (105); and which has a construction wherein a voltage (VDC) on the D.C. side of the inverter (103) is compared with a target voltage (E), a correction signal associated with the difference is added to a speed instruction from the inverter (103) and the speed of the motor (105) is smoothly reduced at the time of power failure with a voltage (VDC) on the inverter D.C. side kept at the target voltage. When applied to a spinning frame or the like, the motor controller of the present invention can prevent non-uniform winding and breakage of yarns at the time of power failure.