To provide a plinth, constructed by cutting one plinth to form a vertical plinth and a lateral plinth having an inclined surface, and making the inclined surface of the lateral plinth abut on a riser not to easily form a gap because the lower end part along the longitudinal direction of the plinth body is provided with an inclined surface inclined upward from the surface side to the back side, and a mounting structure of the plinth to a spiral stairway.

The lower end part 2 along the longitudinal direction of the long plinth body 1 is provided with an inclined surface 5 inclined upward from the surface side 3 to the back side 4, and a decorative sheet 8 is stuck to completely cover the surface 3 and the top face 7 and extend from the top face 7 to the end part of the back 4 and the end part of the inclined surface 5.