A direction restricting device, for the inner and outer tubes of a bicycle seat post, includes at least one axial groove arranged on the wall of one of the tubes, and a direction restricting element fastened to the other tube at the position relative to the axial groove wall. The direction restricting element may be inserted into the axial groove and at least includes an elastic slider provided with an axial opening in the middle. The direction restricting element may be inserted into the axial groove to enable the axial outer walls of the direction restricting element to engage with the axial inner walls of the axial groove to eliminate the interspace between the slider and the axial groove to prevent deflection and wobbling and to reinforce the steadiness of both tubes when the inner and outer tubes rotate relative to each other.