PURPOSE: To adjust seat height on a seat by energizing a seat post slidably inserted in a vertical pipe, by a compression coil spring, and fixing the seat post into a desired slide position by a fastening device provided at the upper part of the vertical pipe.

CONSTITUTION: A pipe-like seat post 4 supporting a seat body at its upper part is slidably inserted in a vertical pipe 2 of a bicycle frame. A fastening device 8 for fixing the seat post 4 by rotating a lever 7 to reduce a space between a pair of lug pieces 6 holding an expanding slot is provided at the upper part of the vertical pipe 2. A pipe body 14 is fittingly inserted in the seat post 4, and a compression coil spring 5 is interposed between the lower end face of the seat post 4 and a large diameter part 16 provided at a lower end part of the pipe body 14. The large diameter part 16 is provided with lock mechanism 22 having a tapered nut 23 fitted into a cylindrical body part 16b integral with the large diameter part 16 and provided with slits 17..., and rotating the nut 23 to screw forward through a bar body 20 so as to enlarge the diameter of the large diameter part 16.