Disclosed is a sliding door self-closing device configured to automatically close a door moving along a rail straightly aligned with an extension rail. The sliding door self-closing device includes a damper configured to move along the rail to automatically close the door and smoothen a closing speed, and a roller assembly pivotably connected to a front end of the damper to move along the rail and provided to be rotatable at the front end of the damper by a predetermined angle. Therefore, it is possible to ensure appropriate driving along the rail even when the rail is deformed. In addition, it is possible to easily remove the damper and the roller assembly through a short space of the extension rail.