A treadmill includes a frame, a support plate mounted on the frame, and a shock-absorbing structure mounted on two opposite sides of the frame. The shock-absorbing structure includes an elastic bar mounted between the frame and the support plate to support the support plate and to provide a shock-absorbing function. The elastic bar is above the frame and under the support plate and has two opposite ends each pivotally mounted between the frame and the support plate. Thus, the elastic bar is movable in a horizontal direction and is elastically bendable toward an inner side or an outer side of the support plate to increase or decrease a contact area of the elastic bar and the support plate and to change the position of the elastic bar relative to the support plate.