Various types of power assist systems are desired in the fields of manufacturing, farming, welfare, etc. In the direct handling which applies force directly to a heavy object, operating force is usually detected by using a force sensor. However, the force sensor is deficient in that, for example, it is sensitive to noise. In this paper, the authors propose a direct-handling enabled power assist system for handling heavy objects in the vertical direction. The system converts the operating force into the angular displacement using a coil spring as a passive element, and detects the direction of the converted force by using a displacement sensor. Any system which coexists and operates with humans is required to ensure high safety. In view of this, the authors will verify the operability of the proposed system, and demonstrate that the system has higher safety in comparison with the systems using a force sensor.