Various power assist systems are used in the fields of manufacturing and welfare. Those power assist systems that handle a heavy object directly by the manual actuating force often use a force sensor to detect the moving direction. However, the force sensor has problems, including one that it is susceptible to noise. In the circumstances, this paper proposes a motor-driven device for supplying constant torque and its application to direct-handling type power assist systems. First, this paper overviews a passive dynamic control (PDC), on which this system is based. Second, this paper shows the principle of detecting the direction of movement not by using force but by using the displacement from the balancing state realized by spring action, which is assumed as the basic state. Third, this paper describes the development of the constant torque device based on this principle of detection, and demonstrates its effectiveness as a power assist system by basic experiments. Since this system may satisfy the fail-safe conditions, it is expected to be used as a high-safety device to make up for the deficits of DD motors.