Commercially available above-knee prosthetic legs prescribed to above-knee amputees are composed of springs and dampers, which restrict the daily movements of transfemoral amputees, such as stair ascent. We propose a novel device that can be attached to multi- or single-axis prosthetic legs and maneuvered actively according to the user’s movements. The objective of this study was to develop an attachable cybernic unit and to confirm the basic capability of the system by evaluation with a stair ascent test. Using a four-linkage mechanism, the device can be operated without precise adjustment of the rotation axes of the prosthetic knee and the device. In addition, the dead point of the system can be avoided by designing a spring-operated dead point prevention mechanism and a thigh-fixing part with freedom of movement. In the stair ascent experiment, we confirmed that using the system, the subject ascended the steps as an able-bodied person. In conclusion, we developed an attachable cybernic unit that can be attached to multi- or single-axis prosthetic legs and maneuvered actively according to the user’s movements, and confirmed that the attachable cybernic unit facilitates difficult movements.