To improve the sealing degree of seals around a push plate.

This slide dumping device for a truck comprises a slidable floor plate 90 provided in a luggage compartment 10 and the slidable push plate 20A provided therein at the upper part of the slidable floor plate 90. On the end of the push plate 20A, a seal 61 is supported and mounted to be movable perpendicularly to the sliding direction for sealing a gap between the end and a guide rail 15. A seal 52 and a backup seal 51 are double mounted thereon for sealing a gap between the end of the push plate 20A and a side wall inner face 14 of the luggage compartment 10 and for backup of the seal 52, respectively. A seal 72 and a backup seal 71 are double mounted thereon for sealing a gap between the end of the push plate 20A and the upper face of the slidable floor plate 90 and in the state of being supported in a vertically movable manner, respectively.