An artificial leg motion assisting apparatus and an artificial leg motion assisting method which are capable of transmitting knee joint motive power to a commercially-available trans-femoral prosthesis without making a wearer feel a sense of discomfort. When the artificial leg motion assisting apparatus is attached to the commercially-available trans-femoral prosthesis, a control unit controls a drive unit by a specified control method on the basis of mechanical impedance adjusted by an impedance adjustment unit and transmits output of the drive unit to a knee joint coupling.
