To obtain a closing accelerating device pushing out a sliding door in the closing direction when the opening keeping state of the automatic closing sliding door is released.

The sliding door 1 is provided with a mechanism in which a movable member 5, which is moved before and behind along a traveling rail 2 by a link mechanism 4, a guide plate 13, to which irregularities are formed, or the like and in which a height place is varied successively with the forward and backwards movement, is secured to a fixing section 3 such as the upper wall, rail or traveling rail or the like of the sliding door 1 traveling on the traveling rail 2 and automatically sealed while the height place is changed and a pushing member 12 abutted against the movable member 5 at a place under an intermediate state is linked and mounted to the sliding door 1 and the movable member 5 is moved in the door-end direction of the sliding door 1.