An improved design of an assistive actuator for the hip joint was proposed. This actuator consists of a speed-up gear train, a flat spiral spring for driving or power accumulation, a differential mechanical clutch, a geared motor, and a two-phase limitless-turn potentiometer. Notably, the differential mechanical clutch prevents the transmission of reaction forces to the motor during the spring winding and release motions, and disconnection can be performed solely by the rotational force from the motor. Additionally, a pair of gears with a shifted phase and extended teeth pitch was utilized. In this research, a calculation method and design conditions for determining the locus of the roll-back motion of gears are proposed. Subsequently, the control system and motion timing for baggage-lifting and walking are presented. Finally, an exoskeletal robot for lower limbs, weighting 64 N and utilizing the proposed actuator, was tested. The differential maximum myogenic potential decreased from 24.1% without the actuator to 6.6% when the proposed actuator was used for the baggage-lifting motion, confirming that the load on the muscles was reduced. Furthermore, in the case of walking, it decreased from 5.4% to 2.2%.