PURPOSE: To automatically open a flap door down to its suspending position, by providing, in an automatic closing device for an upwardly opening rear flap door having a lower hinge, a locking mechanism, a closing mechanism and operating parts connected to these two mechanisms, respectively, so that the closing mechanism ane the locking mechanism are successively operated.

CONSTITUTION: Since a bell crank 26 is in a position sufficiently remote from a horizontal dump shaft 4, a roller 28 is released from a stopper 29 in the intial stage of the tilting of a carrier box 3 so that an actuating lever 30 is moved rearward by the pulling force of a return spring 33, causing a locking hardware 46 to be rotated counterclockwise through the bell crank and a link, thereby the locking hardware 46 is released. When the tilting angle of the carrier box exceeds a predetermined angle, the roller 8B is released from a stopper 10 so that an actuating lever 11 is moved rightward by the pulling force of a return spring 14 and a floating link 20A in a four joint link mechanism 20 is moved leftward for opening a rear flap door 5. Thereby, the flap door 5 is moved down to a position where the flap door 5 is suspended, exceeding the angle at which the flap door 5 is aligned with the bottom surface of the carrier box, due to its geometry.