To prevent the sliding door from violently colliding with the vertical frame of an opening by decelerating the sliding door when the sliding door is automatically closed.

The sill (15) of a sliding door (13) is declined downward toward the closing side of the sliding door. In the sliding door constituted so as to automatically move to the closing direction along the sill due to the own weight when leaving one's hold of the sliding door after it has been opened, an inclined part (23) declined downward from the end (21) of the opening side to the closing side is formed in the sill and a part corresponding to the closing position of the sliding door following the inclined part is shaped to be substantially horizontal (25).