Abstract: A treadmill includes a frame having two arcuate side racks, an arcuate walking board secured on the frame and arranged between the two arcuate side racks, a walking belt surrounding the arcuate walking board and arranged between the two arcuate side racks, a plurality of guiding rollers mounted on each of the two arcuate side racks, and two flexible guide rails mounted on two sides of the walking belt respectively and abutting the guiding rollers, with the guiding rollers rolling on the two flexible guide rails. Each of the two flexible guide rails is provided with a convex portion, and each of the guiding rollers is provided with a concave portion abutting the convex portion of one of the two flexible guide rails.