PURPOSE: To obtain natural riding environment by connecting an elevating frame, always maintaining the support face of a required angle, to the tip of an elevating arm, and mounting a seat on the elevating frame.

CONSTITUTION: A sliding motive power support frame with rollers 11 protrusively provided at the side faces of the front and rear faces is provided, and the rollers 11 are mounted in the longitudinally slidable state along rail frames 8, 8'. Motive power vertical support frames 13 are integrally erected at the front and rear of the center of the sliding motive power support frame, and supporting shafts 14, 15 vertically shifted in the positions are laid across the front and rear of the lower parts of the motive power vertical support frames 13. The lower end of a driving link arm 16 is journalled by the supporting shaft 14, and the lower end of a driven link arm 17 is journalled by the supporting shaft 15. The respective upper ends of the driving link arm 16 and driven link arm 17 are journalled to connecting plates 19, 19 at the rear end of an elevating frame 18 by supporting shafts 20, 21. A prime mover 25 is rotatably connected to the upper ends of the motive power vertical support frames 13 and the tip of a lever 22 connected at a required angle to the lower end of the driving link arm 16, and a seat is mounted on the elevating frame 18.