A muscle force assisting device includes a first attachment arranged along one bone of a joint, a second attachment arranged along the other bone of the joint, and a stretching-artificial muscle having one end provided on the first attachment and having the other end provided on the second attachment, in which an actuator that is extensible by supplying or discharging a substance such as gas, liquid and solid, or a mixture thereof is used as the stretching-artificial muscle, the muscle force assisting device assists motion of a user's joint such as an elbow, a knee and a wrist, a protecting member is provided on the first attachment as a path-restricting section that restricts a path of the stretching-artificial muscle, the protecting member restricts approach of the stretching-artificial muscle toward the joint, the stretching-artificial muscle is prevented from pressing a human body even if the path of the artificial muscle is changed by bending or stretching motion, and the a driving force of the artificial muscle can efficiently be converted into a stretching torque.