A muscle force assisting can increase a bending angle of a second attachment with respect to a first attachment. The muscle force assisting device includes the first attachment arranged along one bone of a joint, the second attachment arranged along the other bone of the joint, and an artificial muscle having one end provided on the first attachment and having the other end provided on the second attachment, an actuator that is extensible or contractible by supplying or discharging a substance such as gas, liquid and solid, or a mixture thereof is used as the artificial muscle, and the muscle force assisting device assists motion of a user's elbow, knee and wrist, the one end of the artificial muscle can be displaced with respect to the first attachment, the one end of the artificial muscle is displaced, and a bending angle of the second attachment with respect to the first attachment is increased as compared with a case where the one end of the artificial muscle is not displaced.