Provided are a prosthesis-mounted action-assist device and a wearable action-assist device that enable step-over-step stair ascending and descending and achieve smooth switching between tasks in accordance with a motion of a wearer. The prosthesis-mounted action-assist device assists an action of a knee disposed between an upper leg frame and a lower leg frame of a prosthetic leg. The device includes an actuator applying power to the prosthetic leg, an absolute angle sensor detecting a hip angle relative to a vertical direction, an angle sensor detecting a knee angle, a floor reaction force sensor detecting a floor reaction force to a wearer, a data storage unit storing reference parameters of phases of tasks, and a control unit comparing the hip angle, the knee angle, and the floor reaction force with the reference parameters stored in the data storage unit to estimate a phase of a task of the wearer and generating an autonomous command signal for causing the actuator to produce power for the phase.
