The present invention relates to a hidden roller of a treadmill. A hidden roller set is pivotably connected at the bottom end of both hollow support arms by means of a pivot, respectively. As a result, the hidden roller sets can be received inside the hollow support arms when the treadmill base is placed in operational state, thereby avoiding an unexpected sliding shift. To the contrary, the hidden roller sets can be taken out when the treadmill base is folded up in storage position. Thus, the rollers are in contact with the ground and work with the fixed type roller set at the rear end of the base frame. Accordingly, the treadmill can be smoothly moved.