A treadmill includes a frame having two sides each provided with two sprockets, two slide rails mounted on the two sides of the frame, two chains engaging with the two sprockets at the two sides of the frame respectively and corresponding to the two slide rails respectively, a plurality of elongate board mounted between the two chains, and a plurality of rollers mounted on each of the two chains. Each of the elongate board has two ends each provided with a connecting member connected with one of the two chains. The rollers on each of the two chains rest on one of the two slide rails. Thus, when the two chains are rotated, the rollers on each of the two chains slide on one of the two slide rails.